SOUTHFIELD, Mich. (AP) _ American Motors Corp., hoping to move into the largest portion of the light-truck market, introduced a mid-size pickup truck line today that it will call the Jeep Comanche.

''The new Jeep Comanche will expand our product line into the largest segment of the light truck market,'' Jose J. Dedeurwaerder, AMC president and chief executive officer, said in a statement.

Dedeurwaerder said the Jeep Comanche pickup was derived from the Jeep Cherokee that was introduced two years ago.

The light truck market in 1986 will account for about 4.2 million vehicle sales, Dedeurwaerder said. Pickups take about 60 percent of this market, or about 2.5 million vehicles, while sport utility vehicles account for more than 16 percent, or 680,000 vehicles, he said.

''Our new line of two-wheel drive and four-wheel-drive Jeep Comanche trucks, combined with our highly successful Jeep CJ, Cherokee and Wagoneer models, puts us into the mainstream of the truck market, with coverage of more than 60 percent,'' he said.

The Jeep Comanche was shown today at news briefings in Detroit, New York and Los Angeles, Dedeurwaerder said. AMC is based in the Detroit suburb of Southfield.