Hard to replace the doors?

[525i winter driver]

Bought this rivnut kit today, so i can just shoot some nuts inside those holes, and fasten the regulator with M4 bolts.

well that ought to make it easy... but i'll answer your other question anyway

I tried to get a nut in behind there, so I could use a bolt, but my hands where to big to fit inside there.. How did you do it?

i had forgotten that - my hands are small and they don't fit either.
iirc - i left the window off the guide and jammed it up into the closed position. i think the filth inside the rubber guides held it up. of the three holes, there is only one difficult one (the highest one). i put the bolt in the hole (head on the inside of the door) before mounting the regulator assembly... i think i used a spanner to hold it in there, it was very awkward but once the assembly was on with that bolt poking through the rest was easy.