So my passenger side rear brake light works fine when the parking lights are off. When the parking lights are on the brake light on that side goes off when the brake is applied.
It's not the bulb, I switched from side to side. I swapped the whole light assembly from side to side, same thing.
Any ideas on what could be the issue?

Thanks in advance as always.

 

Have you checked all the bulbs? Sometimes a wrong bulb somewhere else causes a short.

 

Open the vented black panel in the right rear corner column in the back hatch area of your Disco 2. Inside that access area will be a roughly square electronic part that is supposed to be held onto the frame by a center grounding screw.

Clean and tighten that screw+grounding post.

The above solved all sorts of weird brake light issues for me. Maybe it will help you, as well.

 

2 great suggestions, I'll give them a go this weekend.

Thanks much!