SAme thing happened to my SD, wound up being the window motor. Mine was stuck in the down postion on a 105F day. It was a very hot couple of day I can assure you. There are other things it can be. I would do a "WINDOW" search first on this site, then see a Tech.

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'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)