Anyone tired this? The Onstar antenna jack has an SMB connector, just like the antenna the nav. unit comes with. I read around that some people have successfully tried this.

I take it you're talking about using that gps antenna instead of the one the avics come with? If so, try it worse thing that could happen is the avic won't get a signal. Me personally I've not tried using that since it would keep onstar from being able to know where the vehicle is should it get stolen or be involved in an accident.