75 GMC alternator wiring

[hillytim69]

Hello, my pickup no longer has the original bat light in dash and has only an after market voltmeter. Symptoms are it will not charge at idle and will overcharge at high rpm. I think this is because of the missing idiot light which is part of the circuit to make it work. I can't see any wire under the dash for the light. Nor do i know what color wire to look for.Wiring is a bit hacked into by previous owners over the years. What is the easiest way to make this work? would putting in a one wire regulator solve this problem or can I run a resistor or something? I can't drive it due to it charging well over 15 volts at cruise rpm. All ideas welcomed, thanks guys.