Does anyone know what the fourth pin does on the 4-pin Light Control Rheostat(Dimmer) for the dash lights?

My FSMs only shows the 2-pin model, and i can't seem to find a pin out on the 90-95 FSM.

You may be asking 'why does this fool need to know what the extra pin is for in this plug since there are only 3 wires actually connecting to it?'

Short answer;
in most cars you can turn on the courtesy lights from the dimmer switch(when it clicks), and that is what i want to be able to do.

I can just hook up another switch, but i really don't want to add another switch if i can take advantage of an existing one.

Now for the longer answer:
I am installing another courtesy/dome light at the rear of the 4Wallet. I have also modified the wire to the rear door lock detection switch to activate the interior lights....Open the tailgate, and the lights come on. Now add another deck-light to the opposite side of the rear, and let there be light.

So that is it...any suggestions?
I am not sure how i would test this anonymous pin, since i believe the rheostat/dimmer is switched ground after the bulb in the circuit, and i don't want to fry anything trying to ground the pin to the courtesy light ground.

No i don't have pictures yet(i have been taking a bit), but i will, the entire interior has been removed...another story.

Thanks