The real way to test your problem is to evaluate the voltage drop from the blower neg side to ground. When in high blow the drop should be less than 1v. In theory it should be zero but you are dealing with a transistor amplifier and it does not become a perfect conductor.
The regulator regulates by closing the ground side of the circuit. If you watch this voltage drop it will be over 6v at low speed and drop as the speed increases.
Bosch Master, ASE Master, L1
33 years MB technician