How do I Bench test 73 Swinger gauge cluster

I really shouldn't even try where theres a aftermarket harness but anyway...
First off, majority of it wont work without chassis ground. That blue with white is hot at switch on to power the limiter which in turn powers the thermal gauges ( you don't seem to be concerned with ). In this case the sender wires, grey, solid dk. blue, and purple/violet, not concerned with.
So lights... the illumination bulbs get their power from a orange wire that comes from the headlight switch and park lamp circuit. If illumination is all you desire, 12 volts to the one orange wire connection and a chassis ground should do it.
For the other lights, High beam indicator gets its power from the headlight switch also.
Each turn indicator has a separate wire. Brake indicator requires 2 wires. One is hot at switch on ( I forget where from in 73. That blue with white was the source in some models ). Other wire supplies a ground path through either of 2 remote switches. One switch on the park brake mechanism, another on the proportion valve.
This may or may not help but should explain why there are so many wires attached.