Lights electrical fault!!

Spent most of the day trying to sort what I thought was a simple fault.
300tdi Defender.

When you apply the brakes the front/rear side lights come on.
When you put the side lights on the brake lights come on

I have sealed LED units fitted to the rear (combined side/stop and indicator) and at first presumed one of these was faulty sending a feed back down the loom disconnected both same fault.

I disconnected the rear loom at the multi connector in the engine bay which cured the fault indicating the fault is in the wiring loom going to the rear.
Checked/cleaned earths on rear chassis.
Checked all connectors to rear lights removed insulation to check joints as well.
I have not pulled out the loom that runs inside the chassis but cant see why this would go faulty.
Any ideas?

Would first check all your earths, not just the ones at rear

They can often be the cause of much strangeness

It does sound like a dodge earth.

I.e. the stop and tail bulbs share an earth, sounds like this is back feeding.

Check to see if it happens with normal bulbs fitted. I've had a bad batch of side/brake bulbs back feeding into the sidelights before...