Join Date: Jul 2003
Location: Memphis, TN
Bronco Info: '83 VIN & title on '93 frame/body w/'95 4.9L & EB paint
There's no need to bypass the interlock switch - just permanently install a 12V light across it, and mount the light on the top edge of the t/g inside so it's visible to the driver (in the RV mirror) or when standing at the t/g using the key switch. If the light comes on, the latch isn't fully closed. I haven't tracked down a particular light to use on mine yet, but probably a simple panel-mount incandescent (no LED).
If the light doesn't come on when the switches DON'T work, then the problem isn't related to the interlock switch.
In your case, I suspect a bad dash switch. Pull the L taillight and pierce the heavy LB/Bk wire with a grounded 12V test light to make sure you have power. The green screw for the defrost grid is a good ground. Then check the 2 heavy purple wires (one at a time, of course) while operating the t/g switch in both directions for each wire. If the light comes on, the dash switch for the OTHER direction (from what you're doing with the key switch) is bad. If it lights on both wires, the whole dash switch is bad, or it isn't grounded.
If it doesn't light on either wire, AND the glass doesn't move, the t/g switches are bad, or there's a wiring fault (probably in the t/g jamb where the wires flex).