If you have Onstar and you don't pay for it, or even if you do, this
site tells you how to hook it up to your laptop and use it for GPS.

>From hackaday.com:

this site shows you how to jack in to the gps receiver inside any gm
onstar system. it's as simple as soldering a up a serial cable.
you can then connect to it and either run some gps diagnostic software,
or switch the device to nmea mode so that you can use it with your gps
mapping software in your car pc.

if you've got an onstar system but aren't paying for the service
this might be just the hack for you.

http://www.hackaday.com/entry/1234000170038047/