The Ford Mustangs start moving down the final assembly line at the rate of one every six or seven minutes.

They are ugly. Topless, with gaping holes behind the rear seats, they are sent here unfinished from the Ford Motor Co. plant in nearby Dearborn to be turned into custom-made convertibles on a vest-pocket assembly line at Cars & Concepts Inc.

C&C is one of the nation's two largest companies in the so-called special vehicle industry -- composed mostly of tiny, labor-intensive businesses that make vehicles to order for auto companies such as Ford and for individual buyers.

ASC Inc. in nearby Southgate is C&C's chief domestic competitor. The two companies, with more than $200 million in combined annual sales, are raising what began as a cottage industry to new levels of importance and sophistication.

The companies can do simple modifications such as adding a sunroof. They also can produce super luxmobiles, such as the metallic red, gold-inlaid "Pierre Cardin" Cadillac Eldorado convertible -- with a wooden dash, leather seats, independently retractable glass rear window and a long, sloping front end -- driven by Heinz Prechter, ASC's chairman and founder.

The special vehicle companies are taking advantage of a growing consumer demand for convertibles and other customized cars, vans and trucks. It is an expensive business in which individual buyers are willing to spend $4,000 to $10,000 -- or much more -- to fulfill their motorized fantasies.

C&C can chop as well as top. Today on the Mustangs, the company is topping. The operation is noisy, but orderly. The high-pitched whine of hand-held power tools is rock music to C&C's youth-dominated work force.

The workers rhythmically place vinyl-covered convertible roofs behind the rear seats of the cars. The roofs and their lifting mechanisms are attached. Mouldings are fitted. Extra undercarriage support, usually required in a convertible to ensure body stability, is added further down the line.

Beauty takes form. A completed convertible roof on one of the cars nearing the end of the line is tested with repeated raising and lowering. Form yields function.

The new vinyl top is raised and attached to the windshield frame. The car's windows are rolled up. Then comes the water -- lots of it from all directions at high speed from many nozzles. The artificial rainstorm ends after several minutes. The car is driven off the line, its interior safe and dry. Function works -- and it makes money.

The base prices of 1984-model convertible Mustangs range from $12,214 to $13,815. The base prices of 1984 standard Mustangs go from $7,463 to $10,332. A similar gap exists between the base prices of Chrysler Corp.'s standard and convertible K cars -- which originally were chopped and topped in C&C plants.

Chrysler's convertible program, launched in the middle of the 1982 model year, was so successful that the auto maker is now building the models itself in St. Louis. Chrysler had planned to build only 2,500 convertibles in 1982. Instead, the auto maker built and sold 16,436.

Chrysler was the first domestic auto maker in six years to bring convertibles to market. The company expects to sell 24,000 "rag tops" in the nearly ended 1984 model year.

Everyone from the manufacturer to the dealer gains from the car buyer's desire for something different.

The manufacturer gets money and product exposure because shiny convertibles and other fancy cars on showroom floors attract customers.

And the manufacturer gets all of this at minimum production cost. By turning to companies such as C&C and ASC, auto makers can avoid the cost of extra tools or wages and benefits paid to additional workers.

"Manufacturers can't afford to run a production line at the rate of one car every six minutes. But that's often the rate you need, particularly for low-volume models that require a lot of detail work," said C&C President David L. Draper.

Competition at the manufacturers' level demands high volumes produced at rapid speeds -- at about the rate of a car a minute. That is cost-effective production. But it also requires expensive tooling and equipment dedicated to making one kind of thing.

"We sell flexibility to the manufacturer," said Draper, whose company turns out about 50,000 modified vehicles a year.

Because the conversion companies have relatively few fixed costs -- including dedicated machines and production facilities -- they are better able to respond to lucrative fads such as the born-again convertible craze. The conversion companies also can start fads, such as $17,000 "executive" renditions of Chrysler's base $8,700 "family" minivan.

"We operate in a trendy business," said Draper. "Convertibles may be the rage today, but we can't dedicate everything to convertibles. Our strength is our flexibility."

But both the eight-year-old C&C and its 20-year-old rival, ASC, lately have been pushed to offer more than that.

Auto makers increasingly are demanding that the conversion companies work with them on the research and development of new vehicles and designs. That means development of components and tooling. It means acquiring and using engineering expertise, going way beyond the traditional skin grafting that characterized the work of what has been called the "chop-shop" business.

"The original-equipment manufacturers are beginning to identify special market needs sooner. They are beginning to bring us into the pre-development stages of their products, and that's something that rarely happened in the past," said Patrick W. Logue, ASC marketing and international sales manager.

ASC started out by building electric sunroofs for domestic auto makers. The firm's initials, accordingly, once stood for America Sunroof Co. But the rapid changes in product development and marketing in the domestic and international auto industries have forced ASC to expand.

Now American Sunroof -- which introduced the sunroof to the U.S. car market -- is one of three ASC divisions. The other two are Automobile Specialty Co., which designs and produces special, low-volume vehicles and one-of-a-kind show cars, and Aeromotive Systems Co., which works on advanced automotive programs and projects. C&C has a similar lineup, including its Hurst Performance division, which concentrates on the development, design and production of high-performance technology often used in race cars and expensive sports models.

Of the two privately held companies, ASC is the larger with 1,400 employes and annual sales "over $100 million," according to company officials. C&C executives say the firm employs about 1,200 people and has yearly sales "approaching $100 million."

"Exact figures in that part of the automotive world are hard to come by," said Lee Sechler, director of communications for the Southgate-based Automotive Information Council -- an information clearinghouse for varied businesses in the auto industry. "You have so many small businesses in the made-to-order market. . . . Some of the numbers are overstated or understated for a number of reasons," he said.

But ASC and C&C probably account for 90 percent of the special vehicle work done for domestic auto makers, Sechler and other industry officials said. Hess & Eisenhardt of Cincinnati -- which builds ambulances, hearses and presidential limousines -- is another major player.

Despite the labor-intensive nature of those businesses, all three companies have embraced the computer to meet new design and development requirements. At Hess & Eisenhardt, for example, all drafting and design operations are done on computer-aided design and computer-aided manufacturing (CAD-CAM) systems.

And despite their initial and remaining heavy reliance on domestic auto makers, all three companies are competing for international work against companies such as West Germany's Karmann, and Pininfarina and Ford-owned Ghia in Italy.

ASC recently beat out C&C for the contract to do the modifications on Toyota Motor Corp.'s Celica GTS convertible, which auto industry officials expect to sell well against the Chrysler, Ford, General Motors Corp. and American Motors Corp./Renault models. ASC also is working on the AMC/Renault convertible, scheduled to debut in the 1985 model year.

It is the nature of the specialty-vehicle business that one company may be handling several competing clients at the same time. C&C, for example, is working on Ford Mustangs and Chrysler minivans, and on several other projects that the company refuses to discuss because of client insistence on secrecy.

ASC says that, besides Toyota and AMC, it is working on programs for GM's Buick and Pontiac car groups and for Saab-Scania AB of Sweden. But ASC also has programs under way that it refuses to talk about.

Caution is necessary, C&C's Draper said. "We have to be protective of our clients. You have to keep the projects separate and have restricted entry to places where projects are in development. You can't show the people from GM something that Ford is developing, and hope to stay in business."

The idea is to keep coming up with ideas and unusual designs and modifications that will keep people flowing into showrooms even in economic downturns -- "especially during those times," Draper said.

He said C&C tries to keep its ideas and products tasteful, even though there sometimes is pressure from manufacturers or individual clients to come up with "something garish."

"Someone once asked us to put high, tufted vinyl roofs on their cars -- you know, something that looked like a big cushion. We just wouldn't do it. And we won't do those crazy paint jobs that you see on a lot of those full-sized vans, either," Draper said. "You gotta have standards."