I had this exact scenario just over a month ago and it was a broken earth wire in the left hand rubber conduit, which is visible once you open the boot. I can't believe VW didn't modify this, as it was also very common on the MK4.
Its a very straight forward and quick job to repair. All you need to do, is carefully cut open the rubber conduit and expose which wires are broken. Then solder in a little piece of new wire, in order to create some slack and then heat-shrink the repair to make it nice and strong. As the conduit was sliced open, I just wrapped some black Gorilla tape around it.
Be sure to pick up the electrical wire matching the amperage of your broken ones, heat-shrink tubing, and a soldier iron. I picked up the heat-shrink and wire from Halfords. If you are not handy with a soldier iron, just get someone else to do this, but as I like the satisfaction of making my own repairs if possible, I gave mine a go.
It only took 30 minutes to complete the job and all has been working perfectly ever since, including the boot convenience light, which we lived without for 2 years!
