It was kind of neat, this was back in the days when in-car CD players were still young. The CD player mechanics were in a DIN-sized unit, but the tuner, amp, and electronics were in an outboard "under the seat" module connected with a thick multi-conductor cable. There must have been fifty conductors in that cable, it was thicker than most SCSI cables are today.

The tape player was meant to interface with this CD player, it just had a couple of simple wires that went high or low to indicate to the CD player that it was the active device. You could fake it out pretty easily if you wanted to interface it with something else. It was pretty neat, because if you pressed play on the tape player, the CD would go off and it would use the tape. And vice-versa.

Sony also sold a 1/2-DIN equalizer to go with the tape player and CD player, all of them were the same style and they interfaced nicely.

The funny thing is that the theives who stole the system didn't get the "under seat" module (actually it was buried deep inside the dash in my installation), so the CD player they stole didn't work for them.