[Peter Serio]

Since you say these gauges were added to a car that did not come with them from the factory. Don't know if you are aware of this or not but on all 67 and 68 Firebirds, if the car was ordered with rally gauges, BOTH the entire dash and engine compartment wiring harnesses are specific to w/rally gauges. There are some major differences in the wiring on this car especially a car with in dash gauges. Judging buy some of your photos somebody did the conversion rather poorly. If you want this right I would order 2 brand new reproduction harnesses. One for the dash and the other for the engine compartment. As long as the gauges worked before, as you stated (we can hope the dash cluster is in good working order). In one of your photos I can see that both of the copper pathways that feed 12-14+ volts into and out of your amp gauge have gotten hot. The PC looks to be original to this set. I would replace the printed circuit at the same time as the wiring.

It's time to do this right! Make sure that when you order your new wiring you tell them "with rally gauges."

That way you will have peace of mind knowing that all the wiring is brand new.