Maybe it's just from my years of replacing thrown VW pulley belts, but I didn't find the W124 belt replacement much of an issue...if you didn't have to fool with the tensioner (which I didn't)
Was replacing the alternator, so I decided to throw on a new belt anyway.
You can "coax" the belt onto the remaining pulleys with a flat-tipped screwdriver. The belt is fairly wide, so as you help the belt along the different pulleys, rest the belt onto the screwdriver shaft and it will fall onto the last pulley.
Took about ten minutes to do it this way...most of which was figuring out the path...