How many speakers are in the cab of an 87 Caballero and what sizes are they? How many watts are they rated to handle? My head unit overdrives them at pretty low volumes, and I have to roll down the windows to hear it. Upgrade time.

In 87 they came stock with two tiny 3.5 inch speakers in the dask corners. There were never an RPO for a four speaker sound system in 5th gens.

If you are trying to find places to put speakers in your caballero, check this site out, i just found them for my caballero. they're custom fit.

http://www.crutchfield.com/S-bcGyzdtKRu4/cgi-bin/ProdView.asp?a=0&s=0&cc=01&g=182850&id=detailed_info&i=044K101112

Anything to mount behind the seats as well without blocking the spare?

Many speaker boxes will fit behind the seats on the floor with speakers facing upward. Out of the way of the storage space and sounds ok.

Many speaker boxes will fit behind the seats on the floor with speakers facing upward.
That's exactly what I had in my 87 in addition to the stock dash speakers. In my 79 and 80 Elkys, I cut holes in the bottom front of the door panels for 4x10" co-axial speakers and used the same speaker boxes behind the seat.