if the gauges were "rebuilt for 12V", there is no need for any extra resistors added ... there is never a need for a resistor on the sending unit, it should be a 0-30 ohm sender regardless of the voltage .... there is no "power supply" to the sender, it completes the gauge circuit to ground - power to one post of gauge, other post of gauge to sender terminal to supply ground via the variable resistance of the sender
I'd pull the sender out and [with a ground wire from sender flange to dash], manually move the float arm and see if the gauge responds, if not then check the resistance of the sender with an ohmmeter [terminal to flange] as you move the float - it isn't unusual to find factory defective senders .... if the sender is good and the gauge doesn't respond to it, either the gauge is bad, the gauge case isn't grounded to the dash, or you have something miswired
Bill
