[BiGK]

Have you checked with multimeter to see if you have voltage at the plug behind the bulb holder? Have you checked the continuity of wire from the bulb holder to the terminating end? I believe the other end goes to the fusebox in the boot - have you checked if there is voltage there?

As far as I know, (this is dependant on year), the BSI sends a signal to the rear fusebox to turn on the sidelights - which the numberplate lights are a part of... so if sidelights turn on, rear parking lights should turn on.

 

[BiGK]

Apologies, I had a quick check on wiring diagrams, yes the rear licenseplate lamps go to BSI! That's where continuity needs to be checked... but even if wire is damaged, it is going to be very difficult to repair. It is possible BSI is damaged however that is also expensive and annoying to fix.

Have you tried a BSI reset? Sometimes the BSI can lock out certain functions if it detects a fault and if it's locked out, it won't re-enable as it still thinks a fault is present!

My proposal is to use low wattage non-canbus LED bulbs and wire into the sidelights at the rear fusebox. The low load should be just about OK. If not, use a relay to control those lights if you don't want to take the chance This solution should last you until the car dies

 

[BiGK]

Do you have any errors showing up on the dash? I.e, "Rear licence plate light faulty"? If not, then you can use whatever bulbs you want. Canbus safe LED's use a resistor to behave like a filament and these take too much power! (while creating lots of heat). Non-canbus LED's don't have this resistor and are far more power efficient.

It's the power efficiency you NEED. You can't draw too much current through the loom without issues!


BSI reset is quite common and easy... PROCEDURE FOR BATTERY DISCONNECTION (The 3 Minute Rule)

But be aware, it can make some things worse... or better... just depends on what is actually faulty with the car.

 

[BiGK]

That is interesting... when you reset the BSI, it tests EVERY single circuit it can. I assume you haven't used the fog lights recently?

The numberplate lights share a common ground with the lights I believe and there is a common issue with grounds on the rear lights of saloons.... so it could be that!