Did anyone actually get to the bottom of this? My ST has just done the same thing - offside number plate bulb not working. It started as an intermittent fault, only when the weather was wet, and now it's a solid failure.
I took the rear trim panel out on the N/S where the loom comes through and there is 0 Ohms resistance on the single 12v feed to the N/S bulb connector, but 400k Ohms to the O/S connector.
The loom can be disconnected just behind the N/S light cluster but the loom also does the parking sensors and seems to be ~150 euros to replace the loom.
I can see the loom behind the bumper and it's all neatly tucked in place.
Did anyone try fixing the loom or did people just work out
how to run a cable between the bulb holders (without letting too much more water in to the loom)? It looks like it's a bumper off job get the loom out - did anyone DIY that?