I'm trying to do the same thing. I did a small amount of testing with my little light tester...found out that there isnt power going to the door switches. I think it's a lower voltage or signal than what powers the light. I was thinking of just wiring in a switch and just hit that when I want light in the back. Gonna do some testing to see if I can find what wire gets power when the light's on. It's kinda weird since it dims when the door is shut then dims to off after a delay.

 

I think it's getting power from two sources when it's bright, and then only one source (timed dimmer) when you close the door, so it's not full battery voltage, and not as bright.