Another option is an externally mounted unit (glove box, under seat, ect)
Find one with bluetooth controls or you could get any unit you like that has an adaptor for "steering wheel" controls. Then the key would be finding either buttons from a wrecked vehicle with steering wheel controls (my Durango has them) or finding some sort of control panel and making your own switchboard (hope you're good with wires).
IIRC my unit has a bluetooth option (with adaptor) and can be controlled via my iPhone. But this depends on your radio needs and what not.
Also, speaker upgrades are great. I just put some Infinity kappa component speakers in my Durango to replace the factory 8 speaker system and my oh my does it sound good. I picked up my Kappa 60.9 speakers for $130 per set.