Rear left tail lights come on when brake is pressed, but not on when they should be. Bulbs work.. Help!

[matt_ew]

I've disconnected the tail light body and tested all the continuity between there and to the under seat harness (which is fine), then when I re attach the tail light at the back they all come on, then flash once, then don't work again, unless I press the brake! What is happening!? All started when I replaced a simple fog light bulb.

 

[Deaky]

Sounds like it could be a dodgy earth connection. Check all pins and sockets for corrosion. I think the tail lights are earthed on the OEM stud around the passenger rear wheel arch area. Check here too.