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.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician