HI, I finally have the answer, so am posting it to help anyone else as ignorant as I was about this subject: If your sunroof (in my case, W126) won't open, how do you get it apart to attempt repairs? After all, Step 1 in the factory manual is: open the sliding roof 3/4 open. Now, how do you do that if it won't open? Well, here it is:
If the roof has a rear pop-up function, you can push up on the rear from inside the car; the panel should lift about 1 to 1 1/2 inch. Block it so it stays open, and look inside, toward the front of the car, using a strong flashlight. You will see the inside of the headliner; now look at the forward part, where the liner is fastened to the roof panel: the headlining meets the steel panel at a steel brace which reinforces the front edge of the panel. There are four plastic push pins which hold the liner to the front edge of the panel. You can pop these four push pins loose by reaching in with a long, screwdriver-like tool (I used a pry bar which looks like a giant screwdriver), placing it carefully between the metal brace portion of the roof panel and the lining, and twisting. You have to be careful to get the tool in the right place, as close to the steel brace part of the roof panel as possible, or you can damage the head lining. Once you get one to pop loose, you can see what is going on; when you get all four loose, you can lower the rear of the sunroof back in place, then slide just the headlining part of the sunroof panel back toward the rear of the car, and it will slide out of the way of the interior of the sunroof enough that you can access the fasteners and remove the steel outer panel. Then it's just standard sunroof repair from the manual and your Internet advice forums. If you have trouble getting the lining to slide back, it is probably hanging on something, in my case, it was hitting the guide tube for the cable. Just lift the rear of the steel panel again, reach in with a short tool, screwdriver, etc., and push the lining down a little so it clears the obstruction, and it should slide back pretty easily. There is probably a way to make a simple tool of bent sheet steel to grasp the headlining in the area pf the pins from inside the car and pull down to pull the pins loose; if you make one and it works post it for everyone. Once you see it apart you will know what to do.
All credit for this goes to Dave M., administrator of the "BuyMBParts" Mercedes forum, for his posting of Feb. 2, 2006, titled "How-To Tips on W124 Sunroof Repair". Thanks, Dave M.