There were some posts a while ago about hacking OnStar to get the GPS data, I don't think it came through the OBD2 connector though, I think you have to hack in to the OnStar. OBD2 is quite specific to a given purpose, there are extra connections available for manufacturers to do their own thing but I doubt you'll get GPS through it.
I'd stick with the GPS you have and look into locating the antenna somewhere you're happier. It'll work through plastic or glass, how about on the dash or under the plastic dash? Maybe rear shelf? Try it anywhere that there's no metal between it and the sky.
On my car at the bottom of the windshield (outside) there is a plastic piece before the hood starts, my GPS is affixed to the underside of that plastic where it cannot be seen. Works great.