Marketplace competition may accomplish what federal regulation has failed to do for two decades -- force auto makers to install air bags in all new cars sold in the United States.

Daimler-Benz AG, maker of Mercedes-Benz cars and trucks, and Ford Motor Co. took actions last week that could spark the contest.

Daimler-Benz announced that it was installing driver-side air bags in all 1986 models. Ford announced two days later that it will offer air bags as options in its compact four-door Tempos and its Mercury Topaz family sedans.

Both companies had done extensive market and performance testing of their so-called "supplemental restraint systems" before introducing them to a larger audience.

Daimler-Benz offered air bags as standard equipment in its 1985 model 500 SEC cars, which carried a base price of $58,000. The West German auto maker also offered air bags as an $800-plus option in other 1985 cars.

Ford took a more cautious route. The U.S. auto maker teamed up with the National Highway Traffic Safety Administration to provide federal fleets with 5,000 driver-side air bag-equipped, 1985 model Tempo and Topaz cars. Ford also sold 2,400 similar models to commercial fleet operators, mainly insurance companies.

The air bags were a hit. Mercedes-Benz of North America, Daimler-Benz's passenger car sales operation in the United States, sold 15,600 air bag-fitted cars. The least expensive of the 1985 Mercedes-Benz models, the 190 series, started at about $23,000, without the air bag.

The willingness of a large number of customers to spend $800 more for a non-glamorous safety option in a glamorously expensive machine persuaded Daimler-Benz to install air bags as standard equipment in all of its cars, said A.B. Shuman, Mercedes-Benz North America spokesman.

The 1985 Ford air bag fleet yielded another kind of success. Some 200 of those cars were involved in accidents, including 20 serious enough to cause the air bags to deploy. In those 20 accidents, 17 drivers were wearing seat belts, which are regarded as primary restraints in the auto industry.

According to a Ford summary of the accidents: "There were no fatalities, no serious injuries, and only 13 minor injuries in the accidents, which included a head-on crash with a propane tanker. In all cases, the air bags deployed as expected, and there have been no reports by operators of inadvertent deployment."

Mercedes-Benz and Ford, as a result, can be expected to tout their supplemental restraint systems in 1986 model advertising. And that, according to auto safety advocates such as Ralph Nader, is bound to put pressure on General Motors Corp. and other auto makers to produce air bag fleets.

"It's an image thing," Nader said. "GM does not want to be perceived as being behind in technology of any sort. GM people know that Ford is trying to develop an image as a technology leader" in the area of safety.

GM people point out that their company offered air bags as an option on 10,000 luxury cars in the 1970s and that most of those cars were shunned by consumers. But Nader and other critics argue that GM, long on record as opposing mandatory installation of air bags in cars, did not use much advertising muscle to push its air bag program.

Mercedes-Benz is not about to make the marketing mistake allegedly made by GM. Not only is Mercedes-Benz spending money to advertise air bags; the company is also investing a bundle in telling the public about its "antilock braking system" -- computer-operated brakes designed to stop the car from skidding and spinning out on slippery roads during panic stops. Mercedes-Benz has also come up with another safety device for two 1986 models -- remote-controlled "headlight wipers" that remove grit and grime from car lights during snowstorms and other inclement weather.

Ford also is offering antilock brakes. The betting in the industry is that antilock brake systems will become standard equipment, without federal prompting, within a few model years.