I guess they are called "clip nut" , mine were just a "U" shaped piece of metal that "clips" on edge of sheet metal (over the hole you drilled the rivit out of) and then run a bolt through window motor into clip nut. The purpose is you don't have to be able hold a standard nut in place where there is no way to get your hand in the door cavity. Any hardware store will have these in various sizes.

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Good luck man! You could have got those for allot cheaper on ebay. Everyone has told you all the options. Sounds to me that if you don't get it, just find someone that knows how and will at least get them out and install them yourself.