Yeah, I guess you could use a variable resistor to set the proper voltage. Get a wire from a fused connection in the fuse box, run it to a Variable Resistor/Potentiometer then to the unit.

You must match voltage and current requirements of the unit.

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)