Booming customer demand for light trucks boosted domestic and foreign vehicle sales to record levels in 1985.

An estimated 15.7 million cars and trucks were sold in the United States last year, making it the best automotive sales year since 1978, when a total of 15.4 million cars and trucks were sold in this country.

Interest-rate wars, particularly the below-7-percent financing campaigns conducted by domestic manufacturers between mid-August and early October last year, helped spur vehicle sales.

But auto-industry analysts and officials said yesterday that the increasing popularity of light trucks -- pickups, vans, utility vehicles and minivans -- helped the manufacturers set the sales record.

According to preliminary figures made public yesterday by manufacturers, consumers bought 4.7 million light trucks in 1985, breaking the previous record of 4.3 million light trucks sold in 1978.

"Trucks are rising in prestige," said Dennis Virag, director of automotive industry analysis for Detroit-based Ward's Automotive Research. "In the past, a pickup truck was bought only for commercial applications. But pickups are now being accepted as the second car," largely because they tend to be less expensive than new cars, Virag said.

But less expensive does not mean spartan.

Last year marked a high for luxury options built into light trucks, according to Ward's officials. For example, of the 3.6 million light trucks built in the 1985 model year, 61.3 percent had air conditioning. By comparison, 59 percent of the 3.1 million light trucks built for 1984 were equipped with air conditioners.

In 1979, by contrast, 39.7 percent of the nearly 3.4 million light trucks built that year had air conditioners.

Power door locks and windows, AM/FM cassette stereos with graphic equalizers, sun roofs and other luxury items also are being installed in light trucks, industry officials and analysts said.

Manufacturer competition and new products are other factors contributing to increased truck sales, said Harvey Heinbach, an analyst with New York-based Merrill Lynch, Pierce, Fenner & Smith Inc.

Voluntary import quotas that have restricted the shipment of Japanese cars to the United States since March 1981 did not apply to trucks. As a result, the Japanese have had a virtual "free market in the United States" with trucks, Heinbach said.

"The Japanese have been out there aggressively selling trucks," usually without the dealer markups often found on Japanese cars in this country, Heinbach said. The result has been new-truck prices that generally are lower than new-car prices, Heinbach said.

A random survey of 1986 vehicle prices published by California-based Automobile Invoice Service seems to support Heinbach's argument.

A 1986, base-model Ford Ranger pickup truck carries a manufacturer's suggested retail price of $5,993. By comparison, a 1986, base-model Ford Escort car -- the least expensive car in the Ford lineup -- carries a manufacturer's suggested retail price of $6,052, according to AIS.

For cars alone, domestic auto makers last year sold 8.2 million compared with 7.9 million domestic models sold in the United States in 1984. Foreign-car manufacturers sold 2.8 million autos in 1985 compared with 2.4 million imports sold in this country in the previous year.

Of the estimated 4.7 million trucks sold in the United States in 1985, domestic manufacturers sold 3.87 million and foreign auto makers sold 780,000, according to the preliminary industry figures.

According to industry figures, Ford sold 2.1 million cars, up 4.6 percent from last year; GM sold 4.6 million, up 0.4 percent, and Chrysler sold 1.1 million, up 15.5 percent. AMC sold 123,449 cars, down 35.2 percent from 1984.

Company-by-company breakdowns were not available for trucks.

On the "five best-selling vehicles" list compiled by Ford officials (and endorsed by other analysts), Ford's F-series pickups finished at the top with sales of 562,507 units. General Motors Corp's C-series pickups came in second with 476,048.

Compact, subcompact and mid-size cars held the other top spots: GM's Chevrolet Cavalier compact came in third in overall 1985 vehicle sales, with 431,031 units. Ford's Escort subcompact was next with sales of 420,690; and GM's Chevrolet Celebrity mid-size car finished fifth with sales of 363,619 units.