The regulator is located on top of the blower under the cover at the right rear engine compartment. You will need a diagram but basically the regulator gets in the negative leg of the blower circuit. It is a big output transistor with the base being the control signal from the pushbutton control.
That wire is small and will have from 0 to 6v on it with 6v being max blow. If you get the six volts and only lower speed then the regulator is the problem.
One drawback in the diagnosis is that a weak blower with high current draw will shorten the life of the regulator and when you buy a blower you get a regulator. If you do a good job buying you can get a blower w/regulator for near what the dealer will want for a regulator.
Bosch Master, ASE Master, L1
33 years MB technician