Newly acquired 1966 coupe has battery drain, metering from the negative battery term to frame ground I get .15a. Starting at fuse block and working back thru harness I discovered that the battery draw dropped to 0 when the fuel gauge is disconnected. It's my understanding that the instrument cluster should only be powered when ignition is in the on position, if this is the case where could the problem be coming from. I have a shop manual ordered but haven't received it yet.

 

The problem with older cars like these, is previous owners may have done something to cause a problem. Especially with electrical. Also, switches go bad, wiring insulation breaks down etc.

You'll have to chase down every possible source of power through switches and wires going to the cluster, and see where it lies.