[Old Goat 67]

Don't worry about getting a wire on the back of the gauge just yet, find that Tan wire going to the ribbon cable under the carpet and ground it there. I am assuming there is a ribbon cable on a 64, never worked on one there. If necessary, just disconnect it there and then ground the contact where the Tan wire is. Remember Tan, not Brown. Brown is for Park lights. And, other end of ribbon cable should accessible from inside of trunk, comes up from under back seat, connector there.

Ground it anywhere on that lead going back and the gauge should read empty if it is good. If it doesn't then you have to back up towards the gauge itself, maybe even having to disassemble the dash to get the gauge out. It could be tested when on the bench by duplicating Batt, ground and resistance to ground on the Tan lead connection to prove it's functionality.

If it does operate with that ground on that Tan lead, then you must prove continuity to the rear of the vehicle by either ohming it out front to rear or simply temporarily running a new wire from there to the sending unit for a test of operation.

You could always go by Radio Shack and pick up a couple of resistors to hook up in place of the sender to ground for testing. Say, 35 ohms for full, 20 for half full, ground for empty. I would think you probably have good body grounding if the rear lights work properly, as evident in your other thread.

Charles