chances are he already fried the gauge. most of your gauges use voltage regulators that work similar to a flasher circut, it provides intermintent power to circut. he can go to the h.a.m.b. website and find a tech article on retrofitting his gauges. if your a AD truck owner it's a must read. he can also use a zener diode to drop the voltage. diodes work but with zener diodes if voltage input drops output stays constant,diode output drops if input drops (voltage)
