vw sell adaptor looms for this purpose, I managed to find one in a 1.3 in my local scrappy. It plugs into the H4 plug and splits off to another H4 plug and a spotlight plug. Get yourself a multimeter before you start messing about with electrics, they arn't expensive!
Put the dipped beam on and check each pin for 12v, the one that isnt brown and doesn't have 12v is the main beam wire!
However, now is an ideal time to buy/make an uprated headlight loom, you'll get better light output, plus you wont have to chop up any of the factory loom!