Two things about the Runtz.
* It's mounted on one end on the post of the gauge. If it has a circuit board, typical truck vibration could cause a small crack. Epoxy encasing may be needed.
* If I was in the business, I would want the defective units returned to determine what component failed and revise the design.
Voltage surge.
Operating a 6 volt, series wound (if I remember correctly) starter motor with 12 volts works but I don't know if this increases voltage surge. In any case, a protected auxiliary panel/box connected directly to the battery is probably a smart thing to do for providing power for all add-on solid state devices (today's tech electronics).
Power supply info (tech school stuff).
* Dropping resistor = Poor regulation.
* Resistor voltage divider = Basic regulation. Good regulation if input voltage is constant.
* Zener diode = Good regulation. Correct resistor(s) and capacitor(s) have to be calculated.
* 3 pin/leg regulator = Great regulation. Resistor(s) and capacitor(s) require no/minimal calculation. Built-in overvoltage, overcurrent and thermal protection.
