[rich weyand]

You don't need to do the relays. Just run the second wire back there and replace the switch with a DPDT wired for reversing. Just google "reversing switch" and look for the one with the "X" across the back. Then hook both sender wires into the integral electrical switch and use one wire back to the cab as the sender wire direct to the gauge.

Matter of fact, you can probably use the extra sender wire up to the front as the second wire to the valve motor, since you only need one sender wire to the front with that motorized valve.