I have a 69 SS w/ console gauges. The temp gauge works fine when the ignition switch is on. When the ignition is off, any light (ie. brake light, headlight, parking light etc.) makes the temp. gauge needle go up (about 1/4 of the way for each light system which is on). I think it must be a grounding problem, but I can't figure exactly where to look. How can the brake lights be grounding through the temp gauge? Thanks for any help.
I had a similar problem in that my engine would keep running with the key off and the brakes on, it would quit when I let off the brakes. It turned out to be a poor ground connection in the tail light harness. It was the wire that is screwed into the trunk brace inside the trunk that is centered at the rear of the trunk. Good Luck, it is one of the hardest problems to track down.