I was in the process of replacing the alternator on my 93 300E and decided that it would be a good time to replace the belt while in there.
Could not figure out how to slaken the belt tensioner, so (belts do stretch), I tugged at the belt, and there was enough play to pull it over one pulley, and thus, take the rest of the belt right off.
I agree. You need the manual in order to follow the sequence of threading the new belt back on. My Haynes manual showed four configurations, based on which 6-cyl engine I had, and where the accessory components (in different locations for each engine) shared the belt.
The threading sequence took about ten minutes as I carefully made sure I rounded each pulley correctly. One neat thing is that each pulley gives you a clue of whether or not you are on the right track, since the smooth pulleys indicate that the smooth side should be facing the pulley, while the grooved pulleys indicate just the opposite.
I threaded mine from the bottom and worked my way up, so that I could just tug the remainder of the belt over the last pulley...and I'm not a he-man either!
That was four months ago, and no problems with the belt!
BTW, this is a LOT easier with the fan blade and shroud removed!