The rocker switch (on my '86 it is a rocker) forces the light off when the hatch is open, if you choose. If you leave the rocker switch on, it'll shut when the hatch is closed, assuming everything else is good. Similar to the position on the dome light which keeps the light off with the doors open. Very convenient when pulling a mission.

 

I think mine stays on if the hatch is closed. I'll have to check again...

 

Okay guys... this is not brain surgery and is probably one of the most simple circuits in the car.

On the hatch back, there are two switches in the latches (one in each latch). Either can provide a ground to the Cargo/ Hatch light and switch (as well as to the idiot lights and security system).

The voltage comes from the same source that provides the dome lights and the door lights with power.

Hatch open, hatch light switch on and the truck/hatch light is on.

Hatch closed or switch off and the hatch light is off.

That is all there is too it. It doesn't get any more basic

 

The rocker switch is to turn the light off, even if the hatch is open