First off, switch to SIRIUS, it's the better of the two (IMO). Secondly, replace the factory unit , it really isn't too hard to do. I went through FM and wound up switching the signal too many times in my area. I recently switched to the SIRIUS tuner to hook into my head unit and it's crystal clear and I never have to worry about changing the station and the output signal. It's worth it and if XM's customer service is as good as SIRIUS's then all you need to do is call them up and change your tuner serial number and you're good to go.

Hard-lined in is vastly superior to the FM transmission.