If there is a GPS antenna in that unit, then there is a GPS receiver somewhere.
Basically, you'd need to get the signal outputting NMEA data to a serial or USB cable. considering that several members have taken OEM GPS modules with an antenna and used them with PCs, I think this is something that's very doable. I also recall someone hacking onstar, but I don't remember if it was specifically to get GPS signal or not. Search the forums and see what you can find.