The drivers door has a MASTER supply valve that distributes vacuum from the engine (& vacuum storage tank) to the right front door, trunk, & gas flap.
There is yellow/red lines to lock & yellow/green to unlock.
The source vacuum line is yellow. MOST common failure is the vacuum elements leaking(rubber diaphrams).
The elements have constant pressure OR vacuum so 30+ years and you will need new ones.
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)