Time to break out the DVM and measure some voltages at
the rear of the switches. Mine weren't used for a while
and I had to do some wiggling of the wires at the
back of the switches. Contact probably oxidized.
The compressor kicked on, one time, but since the switch(es)
wouldn't enable the solenoids, it wouldn't release air,
hence it wouldn't come back on a second time, as its
tank had a 'full' charge.
Anyway, most likely a less than stellar quality of copper.
In the 'normal' ARB form, I believe the rear won't power
on until the front switch is enabled, or visa versa, but its a start.
I think I have the wiring diagram in the garage.