I would guess we are talking a 140 car.
The regulator is on top of the blower which is easily accessible by removing the big plastic cover (held by lage philips screws) rear right side of engine compartment. Just fold the cover over and leave all the vacuum lines attached.
The regulator controls the ground side of the motor in response to a 1-6v signal from the PBC.
Bosch Master, ASE Master, L1
33 years MB technician