When you turn the key does this not send power to a relay that in turn allows power to the starter? If you are not even getting the click, I would start at the starter relay. Check for power to if ignition to starter on. If the relay allows a start signal, then the solenoid on the starter motor and check for a good earth by connecting up a bypass. All of this is from experience on other cars and not mk3 specific so i'm sure someone can jump in if I've got it wrong.