The fan clutch and fan will have to come off. Stahlwill 8mm allen type socket gets the bolt out of the fan clutch. Special tool keeps the fan from moving. Locks in behind the fan assembly. Some have used a home-made approach for this tool.
19mm bolt in front of belt tensioner must be loosened. 13mm socket loosens tensioning bolt on tensioning rod.
As was suggested above, owners manual has belt routing diagarm.
Check archives. This has been discussed many times.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.