I'm in a similar position - I would like to check the operation of the resonance flpas on my 95 W124 E300D - the wiring for the vacuum valves does go back via the engine wiring harness to the bulkhead connector on the RHS of the car (driving side UK, passenger side US). On the wiring diagram, the wiring passes through no other multi-plugs, so the control unti cannot be too far away from there - either in the bulkhead area, or under the dashboard at that side.

If you do find the module, would you be kind enough to post its location? - if I find it first, I'll post too.