The vacum is create by the engine manifold and a hose is attached at the back of the engine next to the bulkhead. If you follow the brake booster line it will get you close. The hose then goes through 1 way valves and a resoviour inside the front fender before heading off to the door locks, filler plunger and trunk lock.
There's an SLC diagram in this thread that will be similar:
http://www.peachparts.com/shopforum/vintage-mercedes-forum/164518-76-450sl-any-help-gremlin.html#post1277131