You have 2 sending units. One has the sending unit and fuel pump as an assembly. The other has the fuel sending unit and the low fuel light assembly. The dash gauge averages the two. Pull the sending units out of the tank and hook them up to a multimeter to read ohms (resistance). Across the pins of the connector a two seater's units should read from 8-11 ohms when full (float at top) and 157-170 ohms when the float is at empty.
If you get these readings, you either have a bad ground, a broken or corroded wire somewhere to the front of the car, or a bad fuel gauge in the dash. Good Luck!
