No, I understood you, using the antenna is all I want to do. My carputer (Infill G4) has the GPS receiver built in. That center input on the lower right is an MCX (female) jack for the GPS antenna.
The nice thing with the double-din style of the G4 is the clean factory looking install. To keep the rest of the car like that I wanted to use the already existing antenna and not need to run another one outside (like I did in my last car). I realize that almost everyone else lookng at the OnStar sees a way to utilize its existing GPS receiver, I figured my question would be FAR simplier, but I guessed wrong.