There's 3 wires going into the headlight plug. On my 8v, the yellow/black one is for dipped beam, and the white/black one is for main beam, not sure about other versions. If you go on the principle that the one in the middle is for dipped, and the solid black wire will be negative you should be ok.
You need a seperate relay for each as shown in matey matey. Have just made one myself and will be fitting it this week, along with some xenon bulbs I got from ebay.