Sub Tank not working

[the evil twin]

The "sub" dash is illuminated, and the back lighting of the switch, when dash lights up. There's no transfer light at all.

The 'transfer' light in the switch won't illuminate while the 'sub tank' fail light in the dash is on.
You need to reset the sub tank computer by either, disconnecting the battery, pulling the fuses or disconnecting the computer.
Next time you turn the ign on the 'sub tank' light will go out and the computer will then do a self check (takes a minute or so).
If there is no fault teh light will stay off and all will work as advertised.
If there is still a fault the 'sub tank' light will come back on.

If the light comes back on you can either work thru the tests in the manual or get the computer to show the fault codes, again, by following the manual.
It can be, pump, switch computer, sub tank sender, main tank sender or wiring