I just did this on my '67 as I had to replace the alternator/water pump belt and pretty sure it's the same set-up on the later 108s- it's a bit of a pain in the arse but slacken the bolt on the rear (14mm), and the 2 on the front (17mm)- they just need slight slackening- then use the 10mm tensioner bolt/nut sticking out the side of the pump towards the fender to gain the slack. You should then be able to gain enough slack to lever the pump and get the belt off. You may want to turn the pump pulley to help get the belt off and the new one on. The manual states to use the 10mm tensioner to set the belt tension then tighten the bigger bolts once that's done.

__________________
1993 320TE M104
---------------------------------------------------
past:

1983 230E W123 M102
1994 E300D S124 OM606 (x2)
1967 250SE W108 M129
1972 280se 3.5 W108 M116
1980 280SE W116 M110
1980 350SE W116 M116
1992 300E W124 M103
1994 E280 W124 M104
----------------------------------------------
"music and women I cannot but give way to, whatever my business" -Pepys