The blower regulator is a small black cube a little more than an inch on each side. It mounts to the blower and is no longer available from MB as they redesigned a poor part. Unfortunately the new part requires a new blower which makes the whole process rather pricey.
The cube is actually a large end stage amplifier and is the same as the junk one used on the 140 chassis. Behr has redesigned the 140 regulator and the packaging says it works for 140 and 210 chassis. Who am I to argue with a big company like Behr so I have been modifying the harness and using the Behr regulator on both 140 and 210 chassis.
Bosch Master, ASE Master, L1
33 years MB technician