You might describe the procedure you used to check the gauge with the
sender out of the car, though with 2 of them apparently checking OK,
that's probably not the problem.

Sounds like a wiring problem, but it's strange that you had an opposite
reading on the gauge between the two senders, unless you connected the
non-ground wires at the sender differently each time (ie, switched the
two non-ground wires on the sender posts). You could try switching them
now to see if that changes the gauge reading.

Here's some suggestions for electrical checks. You'll need an
inexpensive multimeter for these; turn the ignition off between test
setup changes, then back on for readings.

Check the fuse panel for corrosion (wouldn't hurt to change the fuse,
too... 10 amp, marked on the fuse block). The emergency brake warning
light is also on this circuit, so it may be possible for that circuit to
cause issues with the gas gauge circuit if it malfunctions.

The dash panel gets ground from a spade terminal on the backside of the
instrument cluster, high up near the center top of the cluster panel;
you might check to make sure that connector is there, with a harness
connector on it, and that you measure zero-ohm connectivity (not
infinity) between the cluster panel and a good frame ground (or a long
wire from the battery negative post) with the multimeter on the "ohm" scale.

Check for zero-ohm connectivity resistance from a good frame ground to
the black (w/white stripe) harness wire connector (removed from tank
sender) to make sure that harness wire has a good ground. Reconnect that
black (w/white stripe) ground wire to the sending unit and now there
should be zero-ohm connectivity resistance between the metal surface of
the tank sender and the frame.

With the ignition switched on, make sure you have 12v at the sending
unit by using the multimeter to check for 12v at the connector at the
tank sender end of the light green wire. That connector normally goes to
the center post on the tank sender, although it may not matter, since
the purpose of the light green and the tan (brown) wire pair is simply
to pass current through the tank sender, which is essentially a variable
resistor (zero to 90 ohms).

Disconnect just the tan wire at the fuel sender, then turn the ignition
on.... the fuel guage should read up past the full mark. Now do the test
again, but this time with that same tan wire connected to a frame ground
with a test lead... the gauge should read empty. If the gauge only goes
partway down with the tan wire grounded, then the fuel gauge MAY have a
problem... it's still possible the wiring harness has a problem.

With the ignition off, disconnect the tan (brown) wire at the guage and
at the sending unit, then use a multimeter and a long test wire to check
for zero-ohm resistance through that wire. Likewise for the light green
wire. Both those wires travel through 2 harness connectors on their way
to the tank, so corrosion at those joints is possible, which could give
a non-zero reading. Before reconnecting them, check resistance of each
end of both wires to a good frame ground.... both should read infinite
resistance, meaning there are no shorts to ground in those wires.