America's growing love affair with light trucks towed the auto industry to its third-best year ever, as car makers reported selling 15.6 million vehicles in the 1988 model year.

The 0.7 percent increase over the 15.5 million sold during the 1987 model year was fattened by strong sales of U.S.-made light trucks, including pickups, minivans and sports utility vehicles.The overall increase came despite a slight dip in car sales, the automakers said Tuesday. It also occurred despite the stock-market crash a year ago.

Foreign and domestic car makers reported car sales slipped 1.4 percent to about 10.4 million cars in the model year that ended Friday from about 10.5 million cars sold during the year before.

But sales of light trucks rose 4.2 percent to 5.1 million in 1988, from 4.9 million in the previous year.

Leading the way were U.S.-made trucks, rising to 4.4 million vehicles this year, compared with just less than 4 million last year, an 11.2 percent increase. Sales of foreign-made light trucks tumbled 24.9 percent to 724,584 vehicles in the 1988 model year from 961,465 in the year before.

"There has been a lot of competition for trucks on the domestic and foreign side," said Jeannette Garretty of the Bank of America in San Francisco. "That has . . . given the appearance and the actuality that you can swing a good deal in that market."

But overall car sales have drooped.