If it's at all like most of the rest of the w126 cars, the pump is located in the spare tire well, just to the right rear of the tire itself. Most likely it is covered by a metal guard plate, and cocooned in a foam "football".

The vacuum plumbing on my cars is located along the right floorboard, near the door sill. To get to it, you have to lift the carpet up, and then the edge of the plastic cover that guards the wiring and plumbing along the floor.

The fastest point I found to test for leaks was to disconnect the line coming from the pump at the junction where it hits the fuel door lock, just behind the trunk carpet on the right side. I hooked up a vacuum gauge to watch the needle swing.

Best of luck, and happy plumbing! :p

__________________
-Josh
Testing the cheap Mercedes axiom, one bolt at a time...