I have a 94' 850 and the Sunroof is only working half way, meaning it will go to the up position, but wont retrack into the roof. I tried to get a good look at the mechanism, buts its almost impossible to see, not to metion I don't know what to look for. Any ideas???? Thanks.

Removing the sunroof and inspect the guide rails:
(Disclaimer: This may not be a precise description, I may have missed steps, and this may not be your problem. Please do this at your own risk.)

Slide the sun shade open.

Take off plastic cover plates from each side of the inside of the roof to access torx screws (I forgot the size).

After removing the screws (I think three on each side) take out the roof from the outside. Do this very slowly and notice how it is connected to the guide rails.

Please make sure to remember exactly how the entire roof assembly is put together, so that you can re-assemble back.

After removing the roof inspect if the guide rails on each sides are ok or not.

Half of my guide rails are broken and I resist to buy rails, $200 each from the dealer. I added some modification to the mechanism it is usable and not perfect. My projected price for the guide rail is $30 each, since I use the sunroof three times a year. So when I see price at that level I may think about it.

Also, I bought the roof shade (it was missing from my car) from RCSR junk yard in CA for $75 (parts+shipping)(for volvo parts: 800-700-volv).