I'd look at the earth connection for the LH light. It would seem the connections good enough for one filament, but not both. Do any other lights come on (front sidelights?) when the brake pedal's pressed?

I took the rear light cluster off again, checked the bulb, both filaments look fine.

All the wiring is coated in a black material and looks fine, no moisture where it shouldn't be.

No corrosion on the bulb holders, all shiny metal.

The wires go through a large rubber grommet and into the boot area, I guess it will be earthed onto the bodywork somewhere in that area.

I will have to have a read up on how to remove the plastic side panel/boot parcel shelf holder thingy, I bet it is awkward.

If you do not want to buy a bulb try a swop with the other side!

Thank-you all for the replies.

I followed your suggestion and swapped the 2 rear light bulbs over and the left side works fine now and the problem is on the right side.

So I have ordered a couple of bulbs, one replacement and 1 spare.

Did you ever clean up the rusty earth connection?

Try running a temporary wire from it to a known good earth.

Thank-you both for the relies.

Tom I was mistaken, it was not rust but dirt where the wires go through a rubber grommet, the wires look to be in good condition.

Elekie&a/c doctor, good idea but I don't think it is possible to take the bulb holders off without cutting wires.

Thank-you for the help elekie&a/c doctor.

You are correct, they are quick release and I swapped the bulb holders over. Now the left brake light and rear light works perfectly, the right one does not.

I had a search for Mark 1 Focus rear light bulb holders. This looks like the one I need.

I'll email them them with the car details to make sure.

www.ebay.co.uk/itm/Genuine-Ford-Focus-MK1-Focus-MK...9

I’ve said it before but you are a genius elecki doc.

Please please continue to stay on The site . Your knowledge is simply amazing !