You'll need to remove the headliner on the sunroof. This is much easier if you can get your sunroof open about 1/2 way. Its only held in place by 3 spring like retaining clips. Insert a long, flat head screwdriver in the front of the sunroof between the headliner and the body. The 3 clips are evenly spaced and you should be able to find them and pry them loose without too much trouble. Once you've carefully removed the headliner (i think its easiest to pull it outside the car rather than inside) you'll have to close the roof. The cable is held to the roof by a clip in the middle rear section. I believe its 2 8mm bolts that hold it in. Once you remove that clip, grab the cable and if the motor still works a little push the "Close" button on the sunroof to feed the cable into the car (careful its really greasy!) If the sunroof motor isn't working you'll have to unbolt the motor in the trunk and then you can probably feed the cable out through the trunk.

When reassambling be sure to grease the cable a lot. You can buy "Sunroof cable grease" from MB but its expensive and you get like 5 lbs of it. I just used wheel bearing grease (about the viscosity of vasoline) and mine has worked great for 2+ years now.