Failed MoT on rear light issue - where's the earth point?

[Laird Scooby]

Originally Posted by Joseph Martin

Thanks folks for all replies so far -

Replaced both with two good bulbs I managed to get, and we are back to the right hand brake light not working when tail lights are on. 3rd level 'middle' brake light works fine.
I'll check re fuses to see if this is the case, I lightly cleaned bulb holders but to no avail yet.

The right hand brake light filament is earthing through the tail-light filament which is why the tail light and brake light don't work with the lights on. You'll also find the brake lights are dimmer on the right than the left with the lights off.

It's a common fuse for the brake lights (or should be), the fault has got to lie in the right hand cluster or connectors. If you can identify the earth connection on the cluster you could run a temporary extra earth to the corresponding earth connector on the left cluster which would prove the earth point for the right cluster is not the fault if there's no change. My guess is either the bulbholder itself or the connector plug as previously described.