The OEM sirius is the way to go and as a previous poster said you can have both the changer and the sirius module at the same time. It is very easy to install yourself. I bought the retrofit kit that included all the extension cables and antenna and it works great. You can hide the antenna in the area where the brake light is if you wish, I did and it works the excellent.

The nice thing about the OEM is that all your controls on the steering wheel work and the presets on the radio itself works. You can clearly see the song information and the sound quality is really good. Tons better than the radio and on par with the changer playing cd's or mp3s.