If it's a resistor pack system, one wire sees ground, one wire sees some voltage. If it's an IC controller, one wire sees constant +12V, one wires sees close to +0V at max, close to +12V at min.

It threw me for a loop when I was diagnosing a blower motor problem and neither wire was a solid ground.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD