2008 Hummer H3 Satellite radio

I live in Toronto and have a 2008 H3X Alpha with the factory navi and XM radio. I would like to change the XM radio to a Sirius radio because in Canada Sirius programming is not available on XM. Sirius makes a tuner (SIRGM1C) which is meant to replace the XM satellite tuner in GM vehicles. The compatability chart for the SIRGM1C says its compatible with the 2006 H3. No mention is made of the 2008 H3 radio and I suspect that this refers to the standard radio anyway and not the the radio with navi. I asked Sirius whether I could install this unit in my truck and they didn't know and suggested that I ask my dealer. I don't want to do that as I suspect that they'll try to discourage me from making the switch by telling me that I would void my factory warranty on the radio and GPS. The installation manual for the SIRGM1C which is avialable online shows a simple installation which involves removal of the XM tuner, mounting the Sirius unit behind the glovebox, and connecting it to the factory wiring harness using a supplied SDAR cable. The diagrams however only show the standard factory radio and not the radio with navi. I've checked with Sirius radio retailers who do installations and they don't know if the Sirius unit is compatible with my radio. If you could tell me whether its compatible or suggest where I might be able to get that information it would be very much appreciated.