Need a Fuel Tank Selector Valve

[wilkin250r]

Only if the motor and diodes also tie to chassis ground through the valve housing (and I doubt that is the case) you could get a ground short.

But if that portion of the circuit is isolated and the only path to ground is made through the selector switch on the dash, then it doesn't matter which wire is "ground". It only matters that one wire is 12V different than the other. It's irrelevant whether you go +12V down to zero, or you go from zero down to -12V, there is still +12V difference.