Landrover Discovery 4 - Brake light not working

Failed MOT due to driver side Brake Light not working.

Have since had a new whole light unit fitted: but it’s still not working.

It is just the brake light, all other lights on the driver side are working fine.
Passenger side - the brake light works intermittently. There’s no flickering.

Please help me! At a total loss here

Hi, might be worth popping over to the 'introduce yourself' section.
On the brake light front a couple of simple checks you can do if you have a multi meter check it has power to that bulb, if it has power then try running a separate earth wire to that bulb, other than that there are plenty of people who know more about the D4 than I @gstuart should be able help with some more in depth stuff if you are electrically minded, all the best.

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What about the high mounted brake lamp? that one is working normally?

D4 brake lights are LED clusters - as above, does the high-level brake light function normally? If the N/S one works intermittently, then clearly there is a fault with the light unit or supply to that one as well. The brake pedal switch signals to the BCM which then switches on the lights (as does the ABS module if ABS / HDC /DSC is active). There are three separate outputs from the CJB to the brake lights, only the high-level one has an intermediate connector so unless there's been any rodent damage, loose connector pins at either end, then it looks like the CJB may be the cause - check for water ingress. There are no fuses as the brake lights are semiconductor switched.