Auxillary input solution.
I've had a Scosche unit installed for a month, perfect solution.
Check Crutchfield.com, 2 products that fit the bill... http://www.crutchfield.com/S-79ELPJW8f7F/cgi-bin/prodgroup.asp?g=251550&est=&search=fm+modulator
Scosche FM-MOD01, Farenheit EFM-01
Both are FM Modulators, not to be confused with an FM Transmitter.
The FM modulator hardwires in line between the antenna lead and the stereo deck, also needs power and ground.
The modulator works similar to a transmitter where it hijacks a preset radio frequency. Because the modulator is hardwired wired in, there is zero external RF interference.
Most FM modulators have RCA in, a simple 1/8th stereo to RCA cable allows you to plug in an iPod or any other MP3 player or audio source with a 1/8th stereo output.
My MP3 player produces CD quality sound through the factory deck when connected to the FM modulator and stereo tuned to 87.9 or 88.3 FM. I have a base stereo with no XM option.
The install also requirs 2 VW style antenna adapters to fit the modulator in/out to the Cadillac antenna plugs. The entire install from Crutchfield cost me $45.00