Which relay switch are you talking about?
The blower is controller by a device called a blower regulator. It works like a huge transistor. The in and out (of the transistor) make the blower connection to ground, based upon the transistor base input signal strength which is a 0-6v signal coming from the pushbutton contoller.
Your description of testing sounds like your are on the wiring to the regulator (without a drawing I don't remember wiring colors).
On 129's I have tested the wiring was at the rear of the fuse box in a single conector.
Bosch Master, ASE Master, L1
33 years MB technician