Rob, the 1990 model with blade fuses should have the low fuel relay in that fuse panel. Try very gently moving the fuel switch between tanks, and see if you can hear the relay click. If it does, you can eliminate wiring between the fuel switch and the battery and concentrate on the wiring between the fuel switch and the Pollak valve.
If the relay clicks, and you can find a tail-light globe or similar with wires on it, unplug the Pollak connector and look for electricity between pins E and D. (Pin F is the one with no wire, E and D are the next two with wires). The light should be on whichever way the switch is set, but might flicker as the switch is changed from left to right. If that all happens, then it is likely that the Pollak valve is faulty.
If the relay clicks, but the light doesn't go when connected between D and E, then the wiring loom between the switch and the Pollak is faulty.
If the relay DOESN'T click, try again to find that missing fuse. It should be in a block of four hanging loose on wires well below the ashtray.
Good luck!