Well, I've been looking at the schematics for other GM-LAN deck equipped vehicles from an online source that my library uses. Namely the 2006 Impala. It has an amplified rear window antenna. It gets it's power from the factory deck. This would imply that the OEM deck decodes the databus to turn the antenna on and off (RAP). I have to insert one of the unused speaker pins into that position and give it a try. I'll have to leave the factory deck on, but I can set it to 0 volume so as to make no noise. I'll probably isolate it with a relay if this RAP signal is present.

If I could post the .pdf you could take a look.

BTW the 2008 is different than the 2007 Express...