There is probably a doubler with rivnuts on the inside of the cabin.  You will be able to access this by removing the headliner (you might be able to reach it through the aileron/flap cable inspection cover).  I don't know if there is a standard arrangement for Comm1/Comm2 antennas.  Could probably jumper one end of antenna cable and test the other end with a ohmmeter.  Hope this helps.

You should be able to drill the screw out.  Use a easy-out set and should be no problem, just make sure you drill in the center of the screw.  There shouldnt be any need to remove the headliner, unless the rivnut or nutplate is not secure.  Here is a picture of what I'm talking about: http://www.toolprice.com/c=awb1XbR0PgJS … 2896W.html
Basically you drill down the middle of the screw about 1/8-1/4 inch, then insert the screw extractor in the drilled hole and turn it out like you would if the screw head were still there.