The basic idea? Intercept the wire that activates the headlights. Most modern cars have relays closer to the battery that send the power directly from the battery to the lights, and the signal wire comes down from the switch.
The little white rectangle with the strike through it is important, thats the coil. Its the circuit that you can activate with a small current, which magnetically pulls the switch for the large current.
Once you get your head around the way that works, there are a number of ways you can use relays.
Another good tip is, if you have an old car, its also possible to insert relays into your headlight circuit so that the actual power for the lights themselves doesn't travel up into the switch, which can wear out.
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