Another Temp Guage Problem

[1976cj5]

This one is unique! My aftermarket temp guage and fuel guage work like they should most of the time. With my park lights on, they still work, but when I turn my headlamps on, the temp guage pegs out to the right (hot). Also with the headlights off, when I apply the brakes, the guage pegs out. I don't know if I am getting feedback and don't understand why it will happen not only with the headlights and not the park lights, but with the brake lights. I installed a Painless wiring harnes when I restored in 2005 and have not had this problem before. I have tried 2 new sending units, with same results. I forgot to mention that it will peg out even with the key switched off and the headlights on.
Also tested both terminals with key off and headlights on and have no voltage between the battery negative and the S or I terminals. Checked grounds and housing is grounded good, and body/frame. I've scratched my head on this one, anybody got suggestions.
When switched on with headlights on or off I have:
12V on I terminal of fuel and between 4 and 5 volts going to I terminal of temp guage.

 

[1976cj5]

I don't know if it is related, but my horn has gone crazy also. I have laid that to a faulty horn switch, but I think it works on a ground, so I don't think it is related, but what do I know?