I to am having this exact same problem. I just got started on this today but I am thinking I have a problem in the wiring harness that goes under the front of the engine. The insulation on those wires is shot. Does your light come on at key position 1? Mine does not. even though it is supposed to be on and then go out when the engine starts. Mine does the exact opposite. Also gets brighter with higher revs. Also tried all the different voltages here and there. Sometimes the whole system would act normally for a few minutes and then I would move the wires and the problem would occur which leads me to think a wiring short. I would like to bypass those particular wires with new ones but I don't know where they come out on the other end. It is a little hard to see where they go once they get to the other side. Mostly because I have no idea of what color they are because the insulation is completely gone.
Please let me know if you have resolved the problem I would appreciate the help.
Vernon Pearce