Greetings!

Does the XM have any inputs? Otherwise, you may need to run the XM via IP-BUS to your radio, and then your radio to the car player's inputs. You would need to rewire / get an amp for that configuration.

The problem sounds like the Pioneer will accept either the IP-BUS or RCA in, but not both at the same time. You need either two inputs into the radio, a switch or you need to daisy chain all three devices in series.

Does your XM radio have an RCA out? Perhaps an RCA switch on the input of your Pioneer...