(Crossed with Andrew's) Have you got a multimeter and the appropriate wiring diagram? Likely faults: Fuse. Locate, remove, inspect. If it has blown it's worth putting another one in and seeing what happens even if you don't know the cause.Switch. Work it a few times. Inspect. Then either see if the 12V feed is being switched to the dip beam output terminal, or check continuity through the switch, or swap the dip and main beam output connections and see what then works.IGNORE: See #5 below. Relay. Locate, remove, inspect the terminals and the base. Probably easier to swap a good one in than check the old one.Jonathan PS: Great description of the fault. The details narrow it down a lot.