Will the cassette player out of my 93 SLE hook up in a 1983 GMC S15 pickup?

you can make anything fit... however the connections on the back of the decks are diff,,, if you have a bone yard around see if you can grab the wire harness outa early 90'* gm so you dont have to cut the one in the other car.. then search the forum and im sure you can find a wire diagram..

I know it won't work on a 93 GM pickup. The head unit is just a head. The main brainbox for the headunit is mounted under the dash above the gas pedal. The head unit area isn't a full-depth socket.

Same on a 94 suburban. Not sure about the 80'*.

Not true on the *-series Bill. Those use the standard GM-size headunits. What you are thinking of are the early component units that came in the fullsize models after their redesign. Sunbirds use the exact same setup.