Owners of Pontiac Fieros, two-seater cars that aren't being made anymore, can get a $500 credit toward the purchase of a new Pontiac, the General Motors Corp. division said.

GM spokesman William J. O'Neill said the company's Pontiac Division was mailing special coupons to 150,000 Fiero owners offering them the cash if they buy any model 1989 Pontiac.The offer expires Sept. 30, 1989, the end of the 1989 model year, and is available in addition to any other incentive the company has offered.

Fiero production stopped in August of this year after sales had slumped badly. The plastic-body car was introduced in late 1983.