Are you saying it isnt engaging at all?
Start checking stuff by having some one hit the switch while you listen at the pump.
If the pump chirps for a second, then stops, you know you have air and no leaks.
If the pump runs on, you have a leak.
If the pump doesnt run on, your axle locks, but the light flashes, check the wires that go into your diff.
If a wire is torn off, the locker will engage, but the light will continue to flash.
If the pump does not run at all check you connections from the switch to the rear pump.