interior lights going off

pop the rear glass, all is ok with all the interior lights.
swing the rear gate open, all the lights go off.
(very annyoing when you're trying to empty the car at night in
a dark driveway)
close the rear gate, all lights come back on.
:banghead:
I see nothing pinched anywhere around the gate hinges or anything.
I've looked from outside, and I've sat in the back and watched as someone else slowly shut the gate.

any ideas?
I'm tired of wearing a headlamp when I empty the car at night.

Liz