I've got the same problem: Rear Doors do not open. Other than "looking", I havn't had the chance to dive into the issue (although everything points to a frozen or broken lock mechanism). Just wondering though, this seems very common with these cars. Had GM Uncharacteristically made a significantly unreliable part? With my 91 ,(according to the former owner) My Passenger side was always inoperable, and then within the last 5 or 6 months, The Driver'* side went on strike as well.
A friend of mine has a 98 GP, which is in shockingly bad shape besides it'* interior. His rear doors have the opposite issue (wont close!), So he cruises around with a Tow-Strap firmly connecting the doors to each other!
I jokingly ask him how his 4door Coupe' is running every so often. :P