As was suggested already, you'd probably do better to replace the stereo altogether. I did this Friday in my '88 740t, and am so glad I did. It had the factory tape player, and worked fine. Despite working, it needed replacing because time marches on, and I don't own any more cassettes, wanted something that could play a CD and something that connects via Bluetooth so I can talk hands-free and play music/podcasts to the stereo from my iPhone.

I did a little reading up, but wasn't prepared for the amplifier issue. I ended up finding a good article and removed the amp and associated wiring. I wired the new stereo, which really wasn't that bad. The most difficult part was getting the power wires properly identified, but now it works perfectly, power antenna included.

Total cost was $145 ($129 for a Sony stereo w/ integrated Bluetooth and CD player, $16 for Scosche dash mount adapter) and about 3 hours of my time.