54 plate mk1 Focus. Built 03. 

Electrical gremlin. Taillights work fine both sides. When brake pedal depressed the right side goes out completely - no brake light/taillight. 

Have checked and changed bulbs, holders are fine. No loose wires at holders. No corrosion. Checked fuses - no issue. Restripped and crimped the joint earth In the boot. No change. Can’t see any damage to wires at top of boot lid.

Next I will change brake light switch (but it does switch the lights on (left/centre).

Any advice from anyone about what else it could potentially be?

I have a knackered abs sensor LR. Could that be potentially causing the issue?

Thanks.

Check the wires where the roof meet the hatch. They break under the rubber cover. Mine went on my old car and the hatch kept popping open when I put the lights on. The wires had broke and were touching each other.

 

Easy ish fix with a soldering iron and some wire.

 

 

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Totally concur.
It's a common problem on mk1's.
It's a bit faffy to get to the wires and the rubber conduit needs pulling away for access.
Getting it back in place after working on the wires is a pain so be prepared to have to pull the headliner around a bit.
I recommend cutting the wires back a bit as they can get corrosion on them which contaminates the copper, some lengths of wire to patch in place is essential plus some shrink tubing plus teva loom tape to seal and reinforce the ares.

Sent from my SM-G965F (S9+)

Does sound like earth problem. If mine I would probably do this which some people will say is a bodge, get some wire. Strip insulation off , about 1 inch, fold over halfway along bare part to have double thickness, wedge it into bulb holder down side of bulb, connect other end of wire to metal car body,

I had something similar on my old mk2. Have you tried swapping the bulb holders over to confirm the bulb holders are ok? I tested mine with a multimeter and it appeared to be ok but there was a fault/corrosion inside the bulb holder. New bulb holder rectified the problem.

1 hour ago, ColW said:

I had something similar on my old mk2. Have you tried swapping the bulb holders over to confirm the bulb holders are ok? I tested mine with a multimeter and it appeared to be ok but there was a fault/corrosion inside the bulb holder. New bulb holder rectified the problem.

+1

I had exactly the same problem on mine. Turned out that it was the bulb holder after all. Go back and try ColW's trick.

ScaniaPBman. 

13 hours ago, hazbaz1984 said:

When brake pedal depressed the right side goes out completely - no brake light/taillight

Classic earth fault. This diagram shows what happens. With the brake switch open the current goes through tail and brake on the side with the break and earths through brake on the other side. When brake switch closes 12V is applied to both filaments on the break side so no current flows.