Yes it has been done. I haven't done it yet. Probably will once I get
it paid off and finish rebuiling my CJ.

Instead of putting a sub in the spare tire you might consider something
like this:

http://www.greensburgautosound.com/p22-23page.htm

Info on speaker upgrade:
http://www.4x4wire.com/jeep/tech/ele.../xj_speaker01/