[davepl]

Normally the power from your battery has to run from the battery junction block to the headlight switch, out the headlight switch and into the dimmer switch, and from the dimmer switch out to the headlight harness and into the headlights.

That's a long run of wire, and they can only use so large of a wire. DC current at low voltages doesn't travel very well to begin with (witness how big your starter cable is vs the power cord of a table saw, which is similarly powered). So, by the time it gets to your headlights what started out as maybe 13.7 volts is now 12.0 volts. That's still enough to power your lights, of course, but not as brightly as if they were connected right to the battery with a short run of cable.

The goal of a relay mod is to get as much of that available voltage (in our example, 13.7) to the lights as possible. It does this by placing a mechanical switch, the relay, right up close to the battery and lights. The 12.0 volts that make it back out of the switch, that used to power the headlights, now do nothing but activate the relay. So, instead of a run of 13 feet (or however long it is from the battery to the cab switch back to the headlights) you now only have a couple of feet of cable, and its usually heavier cable to boot.

Put another way, what used to power the lights now just turns on the relay. The headlight lamps themselves are now directly connected to the battery with just a short run of cable with the relay in between. So they get much more voltage, perhaps 13.5v of that 13.7v instead of a mere 12v.

The relay is just a mechanical switch; you send it power and a little magnet inside closes contacts.

When you have high beams you need two relays. One is for the two low beam headlights and the other is for the high beams. It varies if you have a two lamp (Chev) or four lamp (GMC) setup, but the idea is the same.

FWIW, I purchased a chinese-made "Heavy Duty Headlight" harness from one of the major vendors and it was wired wrong - I had to disassemble it and switch the wires around. Thankfully they are least used green and brown wires so I could match them up to the factory circuits. Ideally you won't have to cross that hurdle. On my other car I made it myself with Radio Shack relays, but if you can get a pre-built harness it's simpler.

Hope this helps. On modern cars MANY circuits are set up like this. The fusebox in my Range Rover has about 20 relays, for everything from fan to fuel pump to even the horn. It's a better setup that not only allows more power throughput, it allows them to save money by using lighter gauge wire throughout the switch side of the circuit that just triggers the relay closer to the actual load.