Gotcha, sorry as there usually seems to be confusion on GPS.
Depending on where your OnStar is located you will need an extension cable.
It has been a while but getting the connector off of the Onstar VIU was not a problem- just don't force it as the cable is not happy if you bend it. the center conductor on mine was about the size of a hair. The outer jacket is more of a tube and the conductor floats in it. I don't remember the fitting on the box because as I said before, there is a short pigtail that went to the module inside. I used the module end as it was the same as my hand held and I had everything apart any way.
You could look for a GPS antenna (I found one for 35$ that had a 20ft cable) That would get you cable and one end already to go then you would only have to add a connector on the other end. Reduces the points to inject errors. Seems a waste to chop it up but saves time and fustration.
Wish you luck, keep us posted
dave
Check this site - they have extension cable connectors and such
http://www.pc-mobile.net/gpsant.htm