Sorry I haven't posted in a while. I was wondering if anybody could help.
Not had much luck on another forum and I think the B7 passat shares alot with the superb. 

My dad has a 2014 B7 Passat estate and the 3rd brake light is constantly lit up. We have removed all of the rear boot paneling looking for water ingress but haven't found any.

When the boot is open the 3rd brake light goes out and brake lights function normally.

I have my dads old Passat B6 Estate and we have swapped out the 3rd brake light but this also stays lit up when connected.

Tonight whilst trying to find the Issue we got the 3rd brake light to work normally for a short amount of time by pressing, wobbling the brake pedal erratically, but then it soon returned.

Possible brake light switch Issue maybe ???? We are struggling to find the issue.

We changed the 2 rear brake light bulbs yesterday and have now ordered a new BLS but any other suggestions would be of help. 

Earth connection. 

Well after much more messing around we discovered that the cause of the issue is the Volkswagen Connect dongle that vw connected up after a recent service. Maybe a nice freebie but money spent by my dad and lots of time lost by us both. 

Came back on again, Grrrrrrrr!