Veteran MB owners will know this stuff, but there was a learning curve for me that I want to share to help the next guy that will inevitably have to diagnose and fix this problem. I took the long way unnecessarily and would have liked to read a post like this one first.

My sunroof activator button sounded like I was being fired at from fully automatic weapons at close range. The reason this happens is the gears are misaligned. The sound is the gears running into each other.

Your particular case may vary but the reason for the misalignment is the motor itself is bolted to the roof with rubber bushings originally used as spacers which decay and flatten out over time, resulting in the metal teeth at the motor's center being out of alignment with the cable which receives them (behind the motor) and runs perpendicular to the length of the car inside metal channels to activate the sunroof.

After opening up the upper lighting console or whatever you call it per youtube videos, you'll notice there are two motors. The one with the visible label towards the passenger's side activates the shade. The one that disappears behind the headliner toward the driver's side activates the glass.

Try these things in this order:

1. Remove the most accessible bolt with the round, flat, pancake head near the middle of the assembly with a T20 bit. You'll know it when you see it. Reinstall it with a washer or lock washer between bolt head and motor to re-establish tension and lodge those hidden teeth. Firmly hand-tighten down. Insert your key and turn to second position without starting the car, then test your switch.

2. If the issue persists, do the same with the second-most accessible flat round pancake bolt on the driver's side. Test.

3. If the problem persists, remove the plate that covers the motor housing by undoing its 3 lighter duty T20 bolts. The plate and the lightweight plastic gear it covers will fall, so prepare for that. Visually inspect the gear for missing or cracked teeth; if there are any, there's your problem and that part will need to be replaced. Mine had no cracked or missing teeth but a seam which resembled a crack at the center which turned out to be of no importance. Not sure if the gear is sold separately but would be great if it were. New OEM motors can be had at fcpeuro.com for the mid-$400's. Also visually inspect the receiving cable which is now exposed up by your roof for metal shavings due to your motor grinding. Theoretically you're supposed to clean all those out. Remove what you can; I tried several methods and fell short of a thorough cleaning. Now for the important part. Replace the gear but line it up and push it in so you are confident the inner and outer teeth are aligned to their respective receivers. The gear should be somewhat tightly fitted and flat. Replace the gear plate. Confirm everything s tightened down, and test the button again. If it still does not work I don't know what to tell you. To remove the motor completely, remove both "pancake" bolts plus a third one hidden behind the headliner toward the driver's side at the rear of the motor housing. The only tool I had on hand that could reach that was the ICON mini ratchet tool #58074 with a T20 bit I cut short with an angle grinder.

Piece everything back together. I had the most luck fitting the mirror to the center rectangular groove of that metal bracket first, and then pushing the larger plastic orb into place with the mirror's "neck" trim removed.

This whole experience will give you a great ab workout. Once complete, reset the switch electronically by pulling & replacing the 9th fuse from the bottom in the left trunk fuse box (should be 25A)--or even better press & hold the button down for five seconds each in the following sequence: up, rear, forward, down.

 

The car is a ‘08 E550 with pano roof. Bought with a nonfunctional sunroof so I jumped in to assess and fix. The sunshade opened and closed w/o issue. The glass roof would not open, the interior lights dimmed when the button was pushed to activate the roof but no response from the motor. The existing motor housing was warped and the metal gear jumped resulting in the roof to open, manually, one side ahead and higher than the other. I was able to close the roof before removing the old motor.
Installed a new MB motor, cleaned and lubricated the cables, rubber, track etc. Manually opens and closes smoothly now with no friction and is synced on both sides. I pulled fuse #9 3x’s and tried the reset method holding the switch for 5 sec’s but the motor still won’t respond. The lights in cabin dim and I hear a click but no response. At this point I’m out of ideas and appreciate any suggestions.