My '93 940 got jammed and each rail wasn't in sync with each other, a quick realign fixed it. It was a much easier fix than I thought it was going to be.

1) Take off that cover in front of the sunroof. I believe it has 2 hex/torx screws.
2) Cut away as little fabric as you can to be able to have access to the motor. You will need to undo (I believe) 3 bolts/screws. Wiggle the motor free and let it dangle by the wire.
3) Turn the key on to the 2nd position and run the sun roof all the way back, for about 5 seconds I'm going to guess. It should stop. Then run the motor forward again, it should stop in about 5 seconds again (this would be where the sunroof is completely closed). Then run the motor forward like you would to open the vent. Then close it again (only a second or two).
4) Put the motor back into the tracks and bolt it back up. Try out the switch again and it should open and close like normal.

It may take a couple times to get it perfectly realigned.
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'93 Volvo 945 w/128K miles