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I don't fully understand the need for the headlight relays. On my setup I installed the same bulbs and everything worked without any problems. Is it because I'm still using the factory harness?
There is no absolute need but the main advantage of the relays are to take the heavy current load out of the old headlight and dimmer switches. Dimmer switches can be easily replaced with new but with the headlight switch being old and probably not in prime condition, reducing the load could lead to a longer life. By using the original signal and wires to energize relays instead of bulbs several amps are removed from the switches.
A secondary advantage is to provide a shorter and larger gauge length of wire supplying the bulbs thus making them brighter. By adding relays nearer the bulbs and having a direct separate and protected source of power, the voltage to the bulbs will not be subject to the amount of voltage drop that would normally be present in the original system.
With the factory loom the voltage goes from battery connection at starter solenoid terminal to headlight switch with the wire feeding other items as well. Once lights are on it is out the switch and thru dimmer switch and then to bulbs. Because of the loom routing, that path approximately doubles the length and uses in some cases a fairly small wire gauge. Adding a larger wire with its own CB directly from the starter to the relays and then continuing with the shorter length of original wiring to bulbs would cut that entire path by almost half so less voltage drop.
It may not be as noticeable on 12v but on 6v, providing the bulb grounds and everything else is also good, there has been a demonstrable difference. A poster on one forum used a light meter under controlled conditions in a darkened garage. Did a test with and without relays. I forget the number but he reported a change of several lux.
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