i vote confirm the source. remove the a-pillar trim and see if it still squeaks. if not, replace in stages. this is NOT a common problem. I have driven many e-bodies, and IMO the A-pillars do not normally squeak.
I've fixed a few rattles and squeaks on my 71 with some type of shim between the offending parts. A strip of rubber, some stick-on foam padding, a piece of moleskin, etc..