Wire from switch to gauge is working, as is the 5volt pulse power supply since other tanks gauge is working.
So problem will be in the switch or it's relay, or wire from switch/relay to that tank, or sending unit in the tank has failed.
Not sure what setup will be on your '88 you will need to pull out the switch to see if fuel Sender wires are switched there or at a relay/solenoid in the frame rail.
If the switch just has 2 wires then fuel sender will be switched at a relay/solenoid, where the fuel lines are switched.
Stock Ford setup should have 6 wires at the switch
Yellow/white stripe wire goes to the gauge
Yellow/blue comes from rear fuel tank sender
Blue/yellow comes from front tank sender
Use an OHM meter set to 200 or 1,000ohms
Ford changed around the OHMS used by gauges and senders in '89, 16ohms(empty)-158ohms(full)
'88 and older should use 73ohms(empty)-10ohms(full)
Unplug switch connector
Put one of the meters probes on a good ground and the other probe on one of the sender wires, it should read between 10 and 73 ohms, higher(or N/C, no connection) means wire is broke or sender is bad.
If you rock the truck you should see the ohms change as that sender's float moves up and down with the fuel in the tank, if ohms just stay at 73 or near there then sender is good but float has fuel in it and sunk to bottom so will always read empty.
There is a small chance the ground wire for that sender is bad but both senders usually share the same ground, so.....small chance.
If both senders read correct OHMs and ohms change when truck is rocked, then switch has a bad contact inside and will need to be replace.
The same switch was used in pretty much all Ford models with dual tanks, so if not available new then wrecking yards should have a bucket full.
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