I have a problem where the left license plate light works, but only when the trunk is slightly open. When it's closed the light goes out. I am assuming that there is a microswitch, or a tilt switch somewhere. Could anyone tell me which, and where it is?

The right hand light works fine. So it looks like there are two switches, one for each side.

Thanks in advance!

 

The operation of the licence plate has nothing to do with the trunk being open or closed when the headlights are on. Mine stays on in my xj8 when the headlights are in regardless if the trunk is open or closed. I just checked.

Sounds like a loose/rusted/corroded bulb socket or bulb.
Since these are exposed to all the crap that comes out from under your car, they can dry out and crack over time. Ever look at the stuff/dirt UNDER your licence plate? Most people miss that.

 

Checked the electrical guide and they're both wired in parallel and go directly to the lighting module, no trunk open switch. If one is working and the other is sporadic, it sounds like a bad bulb, socket, or maybe the wiring running to the socket, as Rpach said.

 

It's worth you checking the wiring where it enters the boot lid from the main loom, where it follows the boot hinge it's known to break here. This is why people often have no number plate lights or their interior boot lights stop working.

 

I'm betting it's the wiring. Mine did the exact same thing. It was the wiring on the right trunk hinge. There were 5 broken wires. Repaired those and license plate and trunk lights now work.

 

Thanks for the replies, very helpful. I'll check out the wiring then!

 

Just got around to fixing it. 4 broken wires at the base of the right hand hinge, where they are bent in a really tight radius.