Agree electric/vac switchover valves. They shouldn't be too difficult to access.

The black vac line into the passenger compartment is only a "vent". I pulled the instrument cluster and bottom panel drivers side off and verified.
What is the logic behind a vacuum "vent"?
Venting vacuum to the lower valve of the VCV makes its vent function inoperative. Mvac test from the line out of the engine switchover valve results in zero vac three way hookup. A vacuum gauge reading from the three way black line out results in zero vac engine running. If the guage is the end point, not vented, 2 inches vac at idle and 5 inches with rpm around 2000. It's the way the switchover valves work. The purpose of vacuum attach lower VCV is to cause an abrupt drop in vac to the transmission modulator valve causing the transmission shift to be crisp.
An open vac will also cause less vacuum for the auxiliary functions such as doors, climate, engine shutdown.
MB knows best? That's why the car is a vacuum nightmare.
I removed the 3 way and have a vac line direct to the VCV lower valve. The car has enough vacuum leaks without intentional.