I have 92 Bonneville SE and the evaporator core housing located on the firewall seems to be rusting out and the condensation dripping out of the rusted edges. Has anyone else had this problem and whats involved as replacement parts. A/C is not cooling as well as in the past also!

It may not be as bad as you think. It sounds like you have a plugged drain and the condensate is leaking out of the seams in the housing causing rust. Could be worse - if it didn't leak out there, it would be on your floorboards under your carpet. Before you try replacing the housing, (not an easy job) try cleaning the drain hole with a wire and cleaning the evaporator core as described in techinfo:

http://www.bonnevilleclub.com/techinfo/?article=62

If you can clear the drain, and the cleaning gets you cool, maybe you could just patch up the housing where it is rusted.

EDIT: the poor cooling may be just low refrigerant.