This is a weird one.

All lights set to automatic position

Close all doors, lock car: front dome light goes off (good). Both rear lights turn on (???)
Unlock car or open a door: front light comes on, both rear lights turn off.

Hooking up a multimeter gives the same result: positive voltage in the rear when all doors are locked and shut, no voltage when a door is open.

 

weird...does this happen with all doors?

 

Yeah. It seems to be completely independent of the doors because I can control the lights using just the fob by locking/unlocking.

I looked at the wiring diagram, and all three lights share the same wire, so I can't imagine its a problem with the BCU. Maybe a short somewhere along where it branches off to the rear? I'll have to trace the wires tomorrow. good thing I don't have a headliner.

 

Jeff, test the electrical system with a diagnostic tool. Failing that disconnect the battery for awhile (say 15-30 minutes) and then reconnect it as the BCU may have thrown a wobbly glitch but make sure you have your radio/CD/satnav head code handy first before disconnecting the battery.

Have you replaced the interior lamps with LED's recently? as they are often polarity sensitive.

Has this just happened for no reason?

 

I've been running without dome lights for many months because I gutted the headliner. AFAIK, they worked fine before that.

- Yes I put an LED festoon in the front. Tried flipping it, and disconnecting it, no dice.
- For the rears I am using these LED's It's pretty clear to me that red is positive, black is ground, and the Striped purple wire is the positive, black is ground. The cargo dome light grounds to the fixing, so that's even more clear to me. Tried switching it for the heck of it... they won't even turn on in any position

I'm ignoring the solid purple wire as I'm not adding a switch for manual operation.

Trying the diagnostic tool and battery disconnect next.

 

I figured it out.

I removed the front lamp to try in the back, and realized the wires didn't do what I thought.
Black and the striped purple wires are both grounds, and the solid purple wire is always hot. In automatic mode, the BCU switches the striped purple wire for ground. Whereas the black wire is always grounded.

That was not clear to me in the wiring diagrams... the wiring diagrams suggest that both purple wires are positive, one always hot and one switched.

So to wire it correctly to automatic mode permanently, ignore the black wire and use solid purple as positive, and striped purple as earth.