the piece pulled out of the dash is probably the brain for the old unit. depending on its age and features. (id need a model number to tell you)
the old deck might be fine for resale provided you test it and verify it wasnt just a disconnected remote line or brain (more common than you might think, i run into this kinda issue all the time with ppl).
do you have aftermarket sub woofer hooked up ? or just factroy stuff?
4cyl autotragic woes...