First stop might be your build plate in the engine compartment to see if the car came with a radio from the factory (vs dealer installed):
510 Radio Becker Mexico (Olympia with stereo-cassette)
511 Radio Becker Mexico, cassette, full stereo
512 Radio Becker Europa cassette
513 Radio Becker Europa stereo, USA
514 Radio Becker Europa LMKU (LW, AM, SW, FM)
515 Radio Becker AM, FM, USA
516 Radio Becker Grand Prix
517 Radio Becker Monte Carlo
518 Radio Becker Europa stereo
519 Radio Becker Grand Prix, USA