Just bought this Wired Relay for my Sirius S50. With this adapter you don't need to have to use the FM modulator that comes with the stock car kit.
The installation was very easy and has preformed exceptionally well. I drove through Dallas and Houston on my last trip and never had to change channels.
I bought mine at Best Buy for $30 but you can find one online and save a bundle. Its well worth that small investment and makes it even safer while i drive since i dont have to change the channels every time i lose the signal.
Give it a try