Adding courtesy lights on rear doors.

Test the wires leading from these:


(Looks like a Torx head screw holding it?)

If a wiire's got 12 volts only when the door's open, you're in business. A simple 12 volt test probe is all you need to verify. Look like this:



Attach the alligator to a grounded bolt or metal, then probe the wire's connector: if there's 12 volts it'll flow through, light bulb in the probe's handle.

Or a multimeter in DC volts mode.

That said, maybe it's preferable to find a similar wire inside the door, so you don't have run additional wire out along the umbilical between body and door. Not sure if there'd be one on the back doors though.

 

There is just one cable behind this black switch and its the ground.

So you can connect a bulb with a permanent 12v from somewhere and the ground from this switch.