Dave the low level light is triggered inside the rheostat I believe. Once the float drops to reserve it closes a circuit for the warning light.
Another change, Toyota going backwards there IMO.
It does sound like a PC failure now the other parts have checked out, does your fuel gauge drop to empty when the ignition is switched off? If no then at least that still has the stepper motor system, so still needs power to go up or down, given the volt meter stopped working at the same time, your back to an earth issue, electrics p*ss me off!
Regards
Dave