The blower regulator is under the motor, under the wipers on the 124 Chassis. The circuit does have an easy access three wire plug directly behind and above the brake booster.
The yellow wire gives the control signal 1-8v and the red carries the current (which can be measured with an inductive amp probe). We had one the other day that had a froze motor and when it was replaced it seemed to be too low of a speed. We monitored the control signal which rose and fell according to the the blower speed chosen but there was never more than 6amps through the red wire. Just to be sure we checked our shop wagen (91 300TE) and it got around 20amps through its blower. We replaced the regulator and all was well.
Bosch Master, ASE Master, L1
33 years MB technician