Sounds like a cracked fuse to me. There should be a single metal strip style of fuse in a black holder about 3/4 inch by 2.5 inch located forward of the fuse box.
The three wires you are looking at should be yellow red and brown. The red is the main power and comes from that fuse. The brown is the blower ground and the yellow is the control signal. It starts low for low speed say 1v and grows to around six volts for "MAX". The signal comes from the pushbutton controller.
BTW the cracking of the fuse is a wear and tear item. The current warms it and it flexes. The cycling cracks the fuse over time.
Bosch Master, ASE Master, L1
33 years MB technician