what clyde said x2
I have been fighting electrical gremlins since day one w/ my '76 towne coupe.
my latest involves my tachometer going to REDLINE and beyond, while the car is OFF.
The tach. is utilizing the power source from the original cigarette lighter.
.
My bulkhead harness connection was melted when I bought the car, and I thought I had found all the related problems.
I too, purchased another harness from a supposed v-8 car, (turned out to be totally different)
the tach worked fine this summer, except for the occasional spike, (needle would jump for no reason.)
apparently there is something else going on.
you may want to save yourself the headaches, and opt for the new harness.
why was the original harness a mess?, and does the donor OEM harness match EXACTLY?
just throwing in my two cents,
there are a lot of wiring diagrams posted on this site, and the resident electrical guys are really good, and usually really helpful, (time permitting).
good luck, and keep us posted (pics maybe?)
rick the mason