My G35 coupe has a weird occurrence.

If you open the trunk, the trunk light doesn't turn on. If you hit the key fob to lock or unlock the doors, the light turns on.

Similarly, the 3rd brake light often does not work. If you get the trunk light to work or tap on the underside of the trunk, you can get the light to either work, or be halflit.....meaning, not red, but pink.

This is the factory 3rd brake light in question.

I can't really get anywhere yet with this. I understand that it is probably a grounding issue.

The oddity is that what the heck does hitting the key fob do to fix the dome light?

 

Not sure how the '03 is laid out but that sounds like either a grounding point has come loose or the wiring harness that goes to the trunk lid could be failing. Is your 3rd brake light in a wing or on the rear window shelf? (Those do seem prone to failure, I just replaced one in our '05 - PITA). And is your trunk light in the lid or fixed on that car?

Only idea about the unlocking bit is that that maybe that gives it enough of a "jolt" to make that loose ground work... Dunno.

 

My 3rd brake light is in the rear window shelf as is the trunk light. The trunk light is probably about 2 inches further back.

You replaced your wing light or inside car 3rd brake light?

When I say unlock.........I mean the trunk is already open and I simply hit the key fob button and the light turns on.

It must be a ground issue....but I don't get it.

 

Replaced my inside car "high-mounted stop lamp" which meant rear seats out, various body trim pieces out, rear shelf out, just to reach the f***in thing. I only broke a couple plastic bits so I'd call it a successful operation.

My '05/06 service manual shows two ground points involved in that part of the circuit. (You should try to look up your own year though before tearing into it...) My ground connection B29 is under the "C-pillar" trim piece, driver's side of the rear shelf (first picture below) and there is a redundant ground in the same harness which I believe is near the driver's feet (hard to tell from the second pic).

For my year those two ground points take care of many things including fuel pump, back up lights, Bose speaker amp, etc... and the redundant front ground covers power seat switches, seat memory, seat heater, power window, etc...

If your '03 wiring is the same you could look into those but it seems unlikely you'd have two dodgy ground points without a bunch of other stuff failing so this could be a dead end idea.

The high-mount brake lamp and the trunk light are literally next to each other on the harness so you should inspect that section of the wiring to be sure nothing's been pinched or somehow broken. Have you had any audio system mods?

Also possible you have two separate problems - 3rd brake light fail plus maybe just the trunk light bulb...? First thing I'd do is remove and inspect that bulb!

Good luck.