Here's the most promising adapter I found. Note that this is an updated version 2 of the adapter, so don't buy an older version.
Adapter:
http://www.logjamelectronics.com/pacaaigm24.html
Manual:
http://www.pac-audio.com/instructions/aaigm24_v2.pdf
Info:
Operation
The factory radio will perform like before. To activate the auxiliary functions, press the CD/Aux or BAND button. Press the CD button twice if equipped w/CD/AM/FM radio). To select Aux1 press REV, to select Aux2 press FWD. If the vehicle is equipped with factory XM and the factory CD changer, the Auxiliary source will be heard through the Rear Seat Entertainment or RSE mode of the radio. The CD/AUX button will select between Aux1 and Aux2. When equipt steering wheel control “Program” button also toggles between the two aux sources in all modes.
>>>>Note: Only Aux1 can be heard from rear headphone jacks.