Power to tail light problem

[chrisj]

One of my tail lights remains on permanently. When l switch it on it turns off. The other tail light works as it should. Can someone please help?

 

[telectrix]

possible bad earth, maybe on another light in either rear cluster. check for corrosion

 

[Simon47]

Bad earth to light cluster and "something" is pulling the cluster "earth" up towards 12V ? When you switch lights on, the lamp goes out because it's now between 12V and 12V.
Probably not full on and off - more likely dim and dimmer.
You see it a fair bit where a tail light is on but goes off when either brake or indicator is used - with brake/indicator light not coming on.
So first check, measure voltage between earth on the light and chassis.

 

[dmxtothemax]

DITTO ! check the earth wire to the light cluster, I'l bet it's dirty/loose/broken.