First thing you need if you don't have, is go over to MyMopar and download the 2-volume 73 service manual. I believe it's Dodge, not Plymouth
"The wiring path". from ground--to the tank sender---through the sender---out the wire, which goes up into the trunk, up forward in the left kick panel, and there' s a mating connector there for the rear harness. The tail/ turn/stop/ courtesy lamp wiring, backup lamps, and sender wire is in there.
From the kick panel goes up into the instrument wiring harness and goes to the cluster via the multi pin connector(s) which attaches to the circuit board for the cluster
The board traces convey the sender to one of the studs of the gauge.
The gauge is powered by the (various names) IVR, instrument voltage regulator or limiter
if the temp gauge works the limiter is likely OK
AS MENTIONED do a quick test by grounding the sender wire. if that (key on) does not send the gauge to the high side peg, now you get to troubleshoot.
Reconnect the sender wire. Acces the left kick panel and (using the diagrams) find the sender wire. Check that "to ground". It should read less if tank is full, more towards empty. The sender goes from about 23--to about 73 ohms or so. IF YOU READ in that range, consider the tank ground, wiring, sender, etc, OK to that point
Next I would drop the column and pull the cluster
