65 Barracuda gauge problems

I assume you bought one of the electronic regulators ($30 on ebay). It sounds like it may not be working (probably not installed correctly), but not positive since it sounds like you are using a 12 V test lamp instead of a multimeter (cheap or free at Harbor Freight). The Vreg should put out 5 V, which may not light your tester. Also, as mentioned, the case of the dash cluster must have a jumper wire to ground.

Did you follow the instructions with photos for installing the new Vreg? You must open the fuel gage to get at the factory one and slip heat shrink over the wire wound arm to disable it. At least, I did in my 65 Dart. Your fish may have the rallye dash with tachometer and may be different, but I recall still inside the fuel gage, indeed even in cars up to ~1972 with the rallye dash.

Until you get 5 V on the Vreg output, neither your fuel nor temp gages will work. As I recall, it comes out on the spring finger that sticks off the circuit board, where the copper stud from the fuel gage protrudes.