Thanks for the input, Steve.
I double checked and it is a '91. It is a 4-Matic, if that matters. I am pretty sure there have been no aftermarket additions. He's had the car for 5 years and this is a brand-new problem.

For what it's worth, my '93 400E does indeed have the feature that let's me roll the windows down or up with the door key ...

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Steve
'93 400E