580 Super E – Headlights spike Temperature!?

OK folks - I seem to be reading a LOT of problems with the Case wiring... looks like this might not be easy. My '86 580SE is fairly new to me - and I'm working through my list of POFURAR. (read - previous owners F$*'d up repair attempt reversals) When I got it, the only electrical thing that worked was the beacon on the roof - and it could not be turned on or off - it just came on with the ignition. The ignition turned when you turned the key - you had to hold the ignition to start or stop it. - I fixed that today. The rear work lights didn't work. - they do now. The side flashers didn't work - they do now The turn signals didn't work - they do now The tach was very erratic - it's much better now. The hourmeter was sporatic... only time will tell if it's better. The fuel gauge didn't work... and still doesn't So - most of these problems were corrected by removing previous owners attempts to fix, bypass, change or modify any and all of the circuits in the console. I have almost returned the wiring to stock, and oddly enough, most things have returned to working. The temp gauge... well..... it works until I turn the front work lights on. The front work lights work, but as soon as I turn them on the temp gauge SPIKES all the way up to the top. My question is... HOW are the front lights tied to the temp gauge? Both light up properly - and bright. The temp gauge works fine, as long as the lights are not on. Oh, the volt meter also drops to 0V when I turn the lights on. (rear lights only dip it a fraction, maybe 1/2 a volt) Obviously the voltage is not dropping to 0 as the lights come on, and the ignition does not cut out killing the engine. I have checked voltage through all the circuit breakers - there is no drop when the lights are on. Could it be that somehow the lights are grounding through the temperature sender? I would think that would fry a wire - the lights must be about 5 amps each? One of the things I did not fix today was the film on the back of the instrument cluster. At some point in the past, a wire shorted, and melted -into the back of the cluster. The result of this was melting through two of the small copper paths, which I believe lead to the panel lighting. I assume that the panel lighting is tied to the headlights, but not sure how an open circuit could cause the gauges to deflect... am I wrong? (should have the same effect as a blown bulb) the copper ribbons are not shorted to one another. The only other indication of something a bit screwy is that the beacon on the roof causes the tach to bounce. With the engine shut off, and the beacon going, the tach pulses up about 200 rpm each time the light goes (or maybe it's when the light is off?) I can hear the light charging (it's a strobe, not a rotater) and it's timed perfectly. Thoughts? where should I look from here?