I think I have the same setup as thunderbirdy. I have a pioneer premier DEH-P660(kinda old) HU and the separate XM unit that plugs into the back of the HU.
Only thing I never liked about it was I couldnt get the pre-sets to set. I always had to scroll through 200 or so channels.
After about a year I canceled it. Cost way to much and I never really liked the songs they had. I may get it again for a month and see if its any good now but its not worth it since I hardly drive.

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LOST KJ'S
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Current Jeep: 2004 Jeep Liberty
-Atlantic Blue
-Air Ride Rear Suspension ; OME/Rustys Lift ; Powertrax Locker ; Moog Lower Ball-Joints ; 245/70/16 Yokohama Geolander A/T-S's on MOABS ; Flowmaster 50 SUV
Lost #007082
http://www.cardomain.com/id/jeepjeepster