I just got '90 535i and can't figure out how to either fold forward or remove the rear seat to check the stereo/amp/cd changer connections to the battery.

How's it done?

Grab a corner, and give it a sharp and quick pull upwards. It should undo the clip, and then do the same on the other side. If i remember correctly, there are a total of 3 (one of drivers side, middle, passenger side)

DDEM3 is right. Just grab a corner of the rear seat, and pull up hard. The clip will come off and you can remove the rear seat. Not hard, especially after you've taken your dads rear seat out more times than you can count :stickoutt