Take off your existing domelight.

There will be 3 wires

power
ground
and a wire that shows ground when the doors is open

Tap the power
Tap the ground when doors is open

run to back

Wire to a new light (get it at an auto store) or buy a neon tube or something,whatever rock your boat. This way, when you open the front doors, it will light up/

If you want to wire the back to light up, you need to install a pinswitch to tell it to turn on when you hatch is open.