Check out this link:
Anyone try a Terk XMDTLK for XM radio?
I installed it in our 2006 with GPS and RES and it works fine on the head unit. Does not work with the RES, but, with a 10 year old, that is fine with us. We get to control the SAT, he gets to have FM and CD control.
You may want to check with your dealer if the one they install can be accessed by the RES. If you have the AM/SAT button on your radio, then they can install the XM factory modual.