https://imgur.com/gallery/0HkMZ05 that is what I unplugged. The circular thing was already detached from whatever it was attached to. Could that be it?

 

When you unplug the IPDM do the lights go out?

 

At this point I would do two things:

1. Start looking around for ANYTHING AFTERMARKET that's in the vehicle, particularly an aftermarket car alarm that might be tied into the lighting circuit.

2. Open the trunk wire loom COMPLETELY to inspect for broken/shorted wiring, it's a very common problem on the G35. This will include pulling out your entire trunk finishers to get down to the metal below the spare tire, you will see a HICAS control box (if 4WAS equipped) and some other electrical components bolted to the sheet metal bottom of the trunk, you will know when you hit bottom. Look for anything spliced into the taillight harness.

 

Stripped everything out of the trunk. Only did I did see was a wire was spliced to the license plate light but I disconnected it completely with no luck. It does have an aftermarket radio and I think maybe the person before me spliced a wire into the bcm and maybe somethings messing up with that. I decided to take the loss and call my mechanic and it’s going to get looked at tomorrow. Thank you all for all your tips/help I’ll keep you updated!

 

While I was in the trunk look I saw a brown relay back there but I can’t figure out what it’s for. When I unplug it I don’t see anything happen. Does anyone know what that’s for?

 

the only time the lights go out is either when I disconnect the battery, open one of the car doors or activate the door switch, or when I pull the bcm fuse out of the fuse box near the battery not on the ipdm

 

This I why I go cutting the wire from the switch and work my way back, to isolate where the short is without assuming anything.
I would have cut it at the brake switch output..
That way I could verify that the switch was working with the brake pedal by measuring voltage on the switch side of the cut....and on the other side of the cut, whether there was another short to some power source.

Then if the short remained (with brakes lights still on), cut at the place where it comes into the trunk.
Then if the lights went out, I would know that there was a short before the trunk and after the switch.

But then soldering a few wires doesn't bother me.