I know the 70's cars well, but don't know if the locks on your 87 are electric- or vacuum-operated...if it's electric, the obvious first place to look would be the fuse. If it's vacuum, then check the vacuum pump, which I think would be in the trunk on an 80's era MB, but I'm not sure. Actually, even if it's a vacuum system it would still be an electric vacuum pump, so your problem could still be a fuse.
I'm sure someone else who has a 190 can be of more assistance.
Anyone else who can nelp him out here?