You've gotta have four wires AFAIK. Combining the circuits (as they'd be wired similarly) the high and low coming off the dimmer/switch are the "trigger" wires for the relay. There must be a 12V battery feed on one terminal, then a 12V to the lights. In the diagram I posted, 86 would be either high or low wire from the dimmer/switch. 85 is self explanatory. 30 would be from a battery source. 87 goes to the headlight.
