[DesertMK4]

I had a similar problem and finally traced it down. FFR (MK4) rear tail lights have a duel element bulb. It is held in place by a spring. The spring can short out the running lights/ and/or turn lights blowing the fuse. If you test without the bulb in place you may not find the fault because the spring only shorts out when it is compressed. I disconnected both rear lights, then tested by applying 12 volts to each line at the rear of the car. There is a disconnect at each corner that makes it sort of easy to do. The lights will come on or sparks will fly. You could use a fuse in the tester to avoid sparks. I cured the problem by taking the bulb socket apart, finding the small burnt spot, re-soldering the contact part, shrink tubing it for about 2 inches and put it all back together. Fixing takes 15 mins. Finding the fault took 2 hours.