I am not very good looking thru threads so there may already be a solution for this. If not, I have 2 solutions to allow you to use the stock XM antenna on the Cobalt. These solutions work with dual blue XM antenna plug. I don't know if it will work any Cobalts with Onstar. I have a GMC Canyon with Onstar and the XM antenna wire has a single green plug instead of a dual blue plug.
I did this because I got really annoyed using a second antenna when I had one on the car already. Plus the second antenna scratched the hell out of my car. I may have had a bad magnet because it slid on the car at highway speeds when it rained.
For the first solution you can try buying this adapter:
XM Satellite Radio at MyRadioStore - Antennas & Extensions - Adapters, Splitters, and Extensions - Vehicle Factory Dual Lead Antenna to XM/Sirius Adapter
I did not try this because of the possibility of a signal loss. This will extend the run on the antenna line. So I went with solution #2.
The second one works with the Terk XM receiver. I don't know if it will work with other XM receivers, but I know it doesn't work with the one from Pioneer. The Pink antenna receiver cannot be recessed. You also need to mount the XM receiver in the trunk. I had to buy a CD changer cable long enough to put the receiver in the trunk. I found a 15 foot one on E-bay for $15 with $5 shipping.
1.) Since the factory XM receiver is behind the back seat, you need to disconnect the XM antenna and feed it into the trunk.
2.) Using a utility knife you need to shave the wide ridge and one narrow one on the Pink Antenna antenna receiver on the Terk XM receiver.
3.) Mount the receiver near the passenger side rear speaker in the trunk that allows the factory plug to reach. Plug the stock antenna so that the yellow wire is attached to the receiver.
The Blue plug should only fit one way. It fits pretty snug depending on how much you shave off the two ridges. Just to be a little more cautious, I put a little bit of rubber cement on the Pink receiver to make sure it doesn't fall off when I hit a bump. I have had not signal loss and I am very happy with this solution. The reception is just as good as it was with the antenna that came with my receiver. My radio actually gets a better response with this set up. That leads me to believe that the antenna that came with my receiver was bad from the beginning.