Problem with lock on a Pontiac 6000?

[DROS]

Just bought a 92 Pontiac 6000. The driver side lock doesn't work. The key won't even turn in the lock. I've tried taking the inside panel off but its still really difficult to get at the lock or even see it because of the frame is blocking access. I can reach in and move the bar connected to lock by hand. When I fiddle with this a small bit the lock works for a couple of turns but eventually stops working again. Any suggestions on how I can; A. Whats wrong with the lock and B.How I can get at it