At sometime in the last few years VW started using radios that could be interrogated by the ECU, it works fine as long as you use the stock beta/gamma radio, swap it for an after market unit then this line becomes 12v active (should display a engine light but not always) which doesnt effect the car in anyway but if you connect a diagnostic unit it will blow up the diagnostic unit

I know this happens on the 9n polo's (you do get a warning light, that goes off with the harness adapter) and the system isnt used in the double din radio's on the GTI/TDI.

VW themselves have been told to not even test your car if it has an aftermarket radio.

If you phone VW they will say that it damages the car to fit one, all it does is generate a self resetting error code and shows the check engine light, once problem is solved the code deletes itself and the light goes out.

Its VW not using standard ISO wiring even tho they use iso plugs for some of there older factory radios. If fitted properly ie. with a harness adaptor its not a problem

The position the k-line takes on the standard iso wiring is the active aerial power feed which is why it gets fed with 12v

ang then blows up the diagnostic unit.