in the last couple of days it has gotten up to 45 degrees outside and the drivers side door lock still doesn't work. I am thinking that the lock probably isn't frozen. What kind of other problems would cause the lock to stop functioning?
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1988 760Tic - 154,000, 1966 M-B 230 sedan - 98,000

I have an 89 760 with the same problem. For the time being, I just swapped the passenger side lock with the driver side. I tried some liquid wrench in the stuck lock with no luck. I took it to a lock smith and he pointed out that you need to grind (destroy) off the outer stainless faceplate to get the assembly apart than you re-assemble with a replacement. The dealer wants $85.00 for a replacement (same keying). One thing I thought of but haven't had the chance to do is placing the lock in an ultra-sonic cleaner with a solvent. I've got a large one at my disposal but if it works a smaller commercial version for jewelry may work.

Good Luck

Steve

Try carefully spraying some PBlaster into the lock mechanism and let that sit for a day, then try again. When I first got my '89 765t, the only lock that worked with the key was the passenger side. Some PBlaster and some patience got the driver's side working again, although the tailgate one doesn't yet.

-Eric

Are you talking about PB B'laster penetrating catalst? This is what I bought today. Both clerks at the store highly praised it. I put some in the lock tonight. Hope it works! If it doesn't work it will atleast narrown down the possibilities. Clerk mentioned that it has some anti-freeze stuff in it so it wont freeze inside the lock overnight. Thank you for the suggestion!
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1988 760Tic - 154,000, 1966 M-B 230 sedan - 98,000

It the handle jammed or is it not unlocking the lock mechanism to release the door?
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-Brandon at FSU '88 744ti 150k miles

The key will go into the keyhole but wont turn left but will turn a tiny bit right and make a click sound. The inner and outer door handles work. All the doors can be locked from the inside with the button on the drivers side door. This just happened out of the blue. Passenger door unlocks/locks fine with same key from the outside.
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1988 760Tic - 154,000, 1966 M-B 230 sedan - 98,000

I was thinking it might have been some vandles. If you put small bits of nail filings in the lock it would show the same signs you mentioned. I would also check to make sure the key is straight.
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-Brandon at FSU '88 744ti 150k miles

This is something I have also considered. I was thinking maybe someone tried to pick the lock and wasn't very good at it? I have heard of locks being damaged this way. If this is the case it might be covered by my insurance.
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1988 760Tic - 154,000, 1966 M-B 230 sedan - 98,000

i dont rember about the front doors, but the back doors have a clip inside the door that holds one of the rods that operate the lock mechanism. when they get old they tend to "let go" of the rod and hence the rod flexes, and you cannot unlock the door. I had this happen in my 85 765. I had to take the door panel off with the back door closed. Not a pretty thing, but i got the panel back on after i glued the clip brackets back on the bottom of the panel. nedless to say I replaced all the clips in the doors. I thikn there were 2 or three in the back door.