Checking the resistance with an ohmmeter can be misleading, as the switch might easily carry the tiny current of the meter just fine, but fail when under a significant load like the sunroof motor. You could test it by putting the removed switch in series with an old headlight bulb and seeing if it lights the bulb to full brightness. This would be about 4-5Amps, and a good test.

I have cured the problem on power window switches by taking the switch apart and polishing the internal contacts. It is very common to find the contacts burned and dirty. The power sunroof switch looks to me like a similar "rocker" design, in which case you must disassemble carefully as the little ball and spring setup which returns the switch to "off" can pop out and disappear under other stuff on your bench.

I use fine emery paper to clean the copper contacts. I have a 4-ft roll of the stuff, about 2-in wide and it's lasted me 20+ years. Just tear off a piece the size of a matchbook cover for most jobs.

400-grit (or finer) sandpaper would probably also do.
--
Bob: son's XC70, dtr's '94-940, my 81GL, 83-DL, 89-745(V8) and 98-S90. Also 77-MGB and some old motorcycles.