Sorry to jump in on this so late.
Could you check (and perhaps take apart again) the wire for the head unit that tells the head unit that the car's lights are on? It might be labelled "parkers" or "dimmer" or such. If the wiring for the stereo itself is wrong (unplugging the head unit doesn't remove the wiring) it may be related to this. There should be an adapter plug for the stereo, but things like handbrake, parker and reverse might have been hooked up manually and this may be the cause.
The other thing - and actually, you might want to try this first - is a bad connection in the tail lights. Grab a 10mm tek bolt bit, chuck it in your cordless screwdriver. Undo the left tail light (2 bolts) and pull it out (it pulls sideways, or directly away from the car to the left of the car, not to the rear). There are three globes and 2 greyish plugs (mine are always covered in dirt so I never remember the colour). Undo the lot, look for dirt and corrosion, clean as needed. While the left light is out and unplugged, is the right light coming on?
Reassemble the left tail light connections, reinstall. Remove the right hand tail light and unplug it. Is the left tail light lit? Check all connections, clean, you know the drill.
My tail lights go all dicky because of a fault in one of the grey connectors on the left tail light. It might be due to the canopy wiring that the dealer's auto electrician added, so that the canopy has both an interior light (powered all the time, even when the ignition is off) and a brake light above the rear window of the canopy. My guy tapped the left hand tail light wiring for this, and a short in any of this could easily cause your issues.