the SIR-GM1 will not work in 07 and up model years because the means by which components speak to one another in the vehicle has changed. the language protocal communication the devices used to use was GM Class-2 BUS compliant, but now, all current 07+ vehicles use the updated GM-LAN Bus communication, thus making the SIR-GM1 unusable in these new model years.
there are a select few vehicles in the GM/Chevy lineup that stayed with the Class-2 Bus components in the 07 year, such as the Corvette and Silverado, but there were also some vehicles that switched mid-year 06, even though they are listed on the SIR-Gm1 compatibility chart:http://akamaipix.crutchfield.com/Man.../607SIRGM1.PDF
the only good news for 07 model year owners, is that Directed did announce and display a GM-LAN compatibile device at this years CES in LasVegas, but they have made no announcement since when this device will be available for purchase.
either way, you can use the OEM GM XM antenna on the Sirius tuner for a seamless OEM installation, with no exterior parts mounted once the Sirius tuner and interface are installed.