Good stuff.

First. Since I am getting the 5 v to the guage, the polarity is good.

It does seem as if it needs higher voltage: the needle moved in the correct direction when the guage output was shorted to ground, it just couldn't make full deflection to reach F.

I didn't mention this, but in troubleshooting the melted sending unit, I got full deflection to F by shorting the sending unit, with the original reducer in place. This has me thinking the melted sending unit was not high enough power rated.

I think the sending unit goes to zero ohms. Maybe I'll pull it and check. Anyway, zero ohms is zero ohms, having the sending unit in parallel wouldn't cause higher amp draw.

I played with the old reducer. The little wire which is wrapped around the contact arm is in bad shape, and was burned in half where it grounded to case. I unwrapped a loop, and re- grounded it to case, then put power to it, with no load. Made me wish I had an analog meter to watch it. Anyway, looked like the contact was cycling open and shut, I was getting a cycling voltage from 1-3 v.

The Haynes diagram shows the reducer powers all the gauges, I'll take another look at that too.

Also it's a 73 charger, and I think it has the rally gauges.