Unfortunately, there is no real way to accurately diagnose whether or not the deck or amp is bad without actually swapping the pieces out.

I would check all your fuses, and if they pass then the next thing I would suspect in your case is the amplifier, it's the one doing most of the work.

I would source a good used amp (should be cheap) and a new head unit with CD- upgrade upgrade upgrade..

Who uses cassette tapes these days anyhow?

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03 540I (GAK!!)
81 280E (W123)
70 LeMans Sport with pissed off Poncho 462"
1989 Silverado Step Side (under the knife- complete restomod)