I must say this so please don't take offense.

You just need to get in there and figure it out.
Imagine the time it would take to get step by minute step
instructions for something like this. One e-mail at a time.

Also search the archives. There are numerous posts on this various
subject.

If you can't get to something then remove or move whatever is in the way.

Solving mechanical problems is like that. At least it always was for me since about age 16. Took a long vacation from wrench twirling during those company car years. Now I only do those tasks that are not too challenging.

Belt tensioner R&R is a pain in the you know what. It's just so crowded in there. I have had tensioner problems on two different 300E's. The belt tensioner and it's adjuster are tricky. I just pay to have it done. Much easier that way. You are probably talking about max. $150 in labor probably less. I remember having one done at an indy MB shop. He was done in 30 minutes.

Oh, one other thing. Be sure to replace the belt tensioner shock absorber while you are in there.

Steve