Hi there my Discovery 2 break lights have been playing up for a while now. Without sidelights on when i press the break pedal the right hand side will come on as normal but the left hand side will light up the sidelight filament. With sidelights on and break pedal pressed the left hand side won’t come on at all and the right hand side both break and sidelight filaments will light up.
I’ve had the fuse board apart but no issues there, i’ve replaced the earths on all the clusters and made my own earth point on the chassis but no luck there.
Any help would be appreciated i’ve considered running my own wire from the break pedal switch to the clusters to try and eliminate a short in that circuit.

 

Grab a multimeter, some long wire, and a friend and chase all the circuits. The wiring diagram is in the google drive in my signature. Download it and go to work.

 

It sounds like a grounding or wiring issue. Since you've already replaced the earths and checked the fuse board, it's worth inspecting the brake light switch and wiring for any damage or corrosion. The odd behavior with the sidelights could be caused by a short or a bad connection in the circuit. Double-check the connections at the rear clusters and make sure the bulbs and sockets are in good condition. If nothing works, running your own wire from the brake pedal switch might help isolate the problem. 2/2