Your owners manual should contain the routing diagram.

If not, you can draw yourself one.

In any event, have this before going farther.

One of the pulleys is the "tensioning pulley." Use a wrench to swivel it away from the belt. There's a hole in the vicinity into which you can insert something so as to block the pulley swiveling back. Frankly, you can get the belt off simply when holding the pulley away with the wrench, i.e. may not need to "lock" it.

Remove/replace belt.

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Kent Christensen
Albuquerque
'07 GL320CDI, '10 CL550. '01 Porsche Boxster
Two BMW motorcycles