I had the same problem and solved it yesterday. Could not even lock the door anymore. The pin was stuck in open position, but I could here the electrics try to lock. Too hard.
If nothing is damked inside the lock, it is probably hardened grease, which got very hard over the years.
There are several methods:
dry the unit with a hairdryer that all moisture goes out and then try to lube it. Maybe that helps.
I had a spare lock and installed that one and then tried to get the other one functioning again as spare.
I washed it completely in nitrobenzine for a long time so that all old grease came out. They I sprayed it with motorbike chain grease, which is very sticky and does not flow out. Works fine again after this cleaning.
In any case you have to remove the door panel first and see if there is no mechanical problem. I also changed the small grommets which hold the metal rod for the door pin and have a link to another extension in my E32. That also help, as the old plastic grommet was damaged and jammed it.

Soemthing from Johan's E32 page might help you.

shogun >>E32 750iL<< 11/88
7 series Tech tips
http://tinyurl.com/3486c

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