My dad has just bought a 52 plate Quattro Sport TDi, everything is great but the alarm keeps going off, the pictogram on the dash shows the drivers door is open but it isn't. What is inside the door that tells the alarm whether it's open or closed?
It could be the wiring or the door lock module. If you get it scanned with VCDS you will be able to tell if it's failing electronically, ie. bad wiring, or faulty module.
I would say it is most likely the door switch, located in the lock module as this seems to be the case in 99% of cases. I had the same problem with my Bora. Replace the door lock module and this will sort it. You will need a "M8 triple square torx bit" to remove the 2 bolts holding the door lock module to the door frame. Other than that, it's a pretty easy fix. Soak the door frame bolts with WD40 as you don't want to be rounding the heads off these and they will be pretty tight considering the cars age. Do not try to use something in place of the bit mentioned above, if you damage the top of the screw you will be left with no choice but to drill it out. If you ain't got one of these M8 bits, Halfrauds do a nice little bit kit which I bought for doing this job as it is the only small bit set which has them in it.
Plenty guides about showing you step by step with pictures how to replace the door lock module.