I couldn't find my truck shop manual I had for my old and sold 86 truck. I read your questions and info on the ford truck enth. forums. My 2 cents worth, The 66 Mustang CVU has to be grounded, I don't know about the truck one. The power flow on a Mustang is a feed wire from the ACC terminal on the key switch to the 3 gauges, An ammeter or Battery voltage meter does not get that wire feed. And I don't have My old instruction manual From Factory Five for the Cobra Kit car using 80's mustang parts. One of the options was to use the stock Mustang gauges like you are doing in the Cobra instrument panel . and wire them instead of Buying the Smith Gauges (a few more dollors.) The FF cobra instructions also told how to use radio shack led's for panel lights. Years ago I used some Ford truck gages in my willys but was able to use the stock willys cluster and willys light sockets with 12 V bulbs.