FM modulator is about $70 from strathfield or autobarn and plugs into aerial lead and converts output from your second unit to fm frequency which you then select on your existing radio.
You can get straight CD players but haven't seen any for a while, I paid $220 new for dvd head unit which plays CD's/MP#'s and dvd's and added to my existing sony Cassette/tuner and 6 stacker setup, so gives me all possible combinations. The sony combo's can be picked up for under $400 with 6 stacker or head unit (CD/Tuner)only for about $200 (supercheap brochure this week) Fitting is not hard to do your self and if you buy replacement when you sell car take it out put back original and transfer new unit.
Trick is not to cut your existing wiring, the camry runs an ISO plugand adapter harnesses are about 20 bucks.