On my oil and water temp gauges, it looks like the light for each and the tack has one wire coming from the bulb socket. I assume that is 12 volt power so how does the light ground without a second wire. Also, it appears that you have to take to cover off to replace the bulb....what a pain. I added the oil temp gauge and it did not come with a light so I bought a generic one to install. The Super Sun Tach has a light that works but the two gauges have no light. One is missing and the other must be burnt out. I figure now that I have it restored, I might get caught driving it at night. Still trying to fix a few electrical items and I'm not an electrician....LOL!

 

With a single wire one has to assume the ground side would be the case and mount.

 

The metal casing of the gauge (or its mounting bezel if that is metal) should ground to the dash if bolted metal to metal - if however you mount the gauge to a non-metal surface, you should be able to run a jump wire from one of the gauge mounting post bolts to another ring terminal bolted to some other metal part of the dash to create a ground.