One other thing to check, which was a real "mystery fix" for me, is the thermovalve. I had an accidental FWFW repair that got my A/C blowing cold when I needed to replace the thermovalve insert and, instead, just pulled the entire relatively new thermovalve from my 82 and put it in my 86. I was getting nothing but hot air out the vents, replaced the thermovalve and voila! - cold air. Apparently, there is some circuitry in the thermovalve body that doesn't usually go bad with the every 2 years or so monovalve insert replacement, but it certainly helped me. That may not be the problem, but it's certainly worth checking - not obvious.

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1987 300SDL 265,000 mi.
1982 300SD 325,000 mi. (and holding)
1956 Packard Clipper 150,000 mi.