73 Dodge Dart Sport Fuel Gauge Wiring

You should be able to find the single wire for signal sender in the trunk. It goes through a rubber grommet and down to the sender post. The ground is accomplished with a metal jumper over the rubber hose creating a continuous chassis ground path. If that ground jumper is missing you can create one with a small piece of wire held down with a small hose clamp on each end.
Easy testing, first , with the ignition switch on , pull the sender wire off the temp sender and check it with a volt meter. You should see flashing 2.+ to 3.+ pulses of current using a digital meter. Now put that back and look for the same current going on the fuel sender wire.