72 Coupe - My fuel gauge was acting wonky, so I cleaned the contacts at the sending unit, all the grounds in the rear lighting, and the ground at the rear frame. Now, it works great except for when I turn on the lights.

When the light switch is pulled to first or second position, the fuel gauge goes to Empty. 

I'm guessing a bad connection to the ground wire at the gauge, or a bad headlight switch ? Neither will be fun to get to, so which area would you inspect first ?

Well...since you mentioned the ground on the frame at the rear, I would start there. The grounds for the lights and fuel sender are in the same spot. Turning the lights on causes the lights to ground thru the fuel sender, which in turn would think the tank is empty. If you dis-connect the sender, the gauge should show full. As the sender float goes down, it increases the resistance, and moves the needle on the gauge down. I would suspect you disturbed the ground(s) there, and just need to re-visit, and clean. 

Good call, thanks !

I ran a temporary ground from the sending unit to the frame and the gauge reads steady. So I have a little more ground cleaning to do, but you saved me from going into the dash for no reason !

Good to hear! 👍