Had an amusing time this morning in the fog. Driving happily along about 3 miles from home when I encountered some fog so switched front and rear fogs on and the boot lid (not the full hatch) popped open. Parked with all lights and engine running and sent child out (nearside) to shut it assuming he had not done it properly at home, but he couldn't shut it so I tried. Took me 5 mins to suss that I had to turn the rear fogs off before the mechanism would allow closing. Have tested it since and as soon as the rear fogs are switched on the boot opens, quite a useful feature as I can now open the boot without needing to switch off and remove the key! Any one got any ideas - my guess is a wiring problem as wires to both lock and fogs are in the same area? 

Almost certainly a wiring problem, like you said.

Now that is pretty clever stuff

The wires (10 of them) to the boot release buttons and fogs/reversing lights  all are routed through the (hollow) left hand boot lid support.

 

Where the wires leave the support (behind the trim near the heated rear window), the wires can break with age and use as they are forced into a tight curve with every boot lid operation.

 

Before the wires break completely, sometimes the insulation (ordinary pvc,) cracks. I suspect two adjacent wires have cracked insulation and are now shorting. The cold weather makes the insulation stiff, making a break more likely still.

 

The next development could be that you cannot open the boot lid any longer at all (i.e. one of the 10 wires has completely broken).

 

In the meantime, if you only open the whole hatch every time, the problem will probably not worsen. This will allow you time to get it investigated, and still be able to get in your boot!

 

HTH.

Edited 22 January, 201412 yr by CombatWombat

A bit OT perhaps, but I had almost the same problem on my car yesterday. When I removed the key from the ignition it popped the boot lid open. I also realized that opening the driver door popped open the boot.

 

In my case the cause was not hard to find. I had been happily hacking away with VCDS the night before, so it was just a matter of resetting a couple of bits to their original value and the spontaneous opening of the boot was stopped.