If you know where the noise is coming from, why not stop it (or reduce it) at the source, if you can? Isolate/by-pass/filter the 'XM' thingy's power lines. Moving that power source from the vehicles wiring by moving it to the battery isn't really removing it from the vehicle's wiring, just changing where the power happens to be tapped from that wiring. If that 'XM' thingy is putting noise into the wiring now, it certainly can do the same thing from the battery too. Making that move might make disconnecting any 'standby' power easier, but it isn't really 'fixing' anything. Not being familiar with any of the 'XM' stuff, will it loose it's programing by doing that? (I don't know, that's why I'm asking.)
- 'Doc

The XM and the radio are hooked to power at the same location...I was just going to experiment.

The XM cigarette power adapter is the actual cause (it causes issues with the XM off), not sure what else to do but live with it.

The first two things that come to mind is 'live with out it', or gently explain to it what you will do to it with a very large hammer if it doesn't straighten up. After that, it gets $ort of complicated. $ounds like another power supply for that 'XM' thingy might be an option. Don't take that as a good idea, it probably i$n't.
- 'Doc