I think Tiny is right. The problem you are dealing with is common in evaporator housings on old vehicles. After years of dirt accumulation and foam seals that had deteriorated and fallen to the bottom of the case, you'll have to get that crud out and possibly repair the seals.
Sorry for the bad news, but it will be worth the effort in the long run.

I didn't do it, no one saw me do it, you can't prove anything! "Bart Simpson"