I bought a 2004 bonneville a few months ago and finally got around to moving on a small project - to install my 2m/70cm modile rig in it. When I had it in my old van (a1986 chevy sport) I had no problems with growing a large antenna farm but im trying to stay cool here so my plan was to use the through the glass cell phone antenna that is mounted on the back window. This looks factory and I think it may actually be the onstar antenna, although I was hoping it was an after market cell phone antenna. Any idea if my assumption is correct? Even if it is I still want to use it but I need to track down where the coax terminates without destroying the interior. Any suggestions?

Welcome!

This is the OnStar voice antenna. Unless your car has had the Analog to Digital OnStar upgrade, this is an analog only antenna. The coax runs down the right 'C' pillar into the trunk. There is a black plastic trim panel immediately behind the rear seat back (in the trunk). If you remove this, you'll find the OnStar module on the right side diagonal brace. You'll find the other end of the coax connected there.

Further to my last post (you may also be wondering)... OnStar communicates over the cellular network with established agreements between OnStar and a variety of carriers. Therefore, this is the same type of antenna you would expect to find on any 3 watt analog car phone.

Most mobile rigs run 25-50 watts output, some bigger ones like the 857 run 100 watts. Better look into running any serious power thru that antenna setup...or you may be replacing the finals very soon.