i hardwired my existing fog light switch to turn on my driving lights anytime i want like with park lights, low beams, high beams or just the driving light itself as long as i have the key in the ignition (safety feature that is really useful when running late for work..
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a lot of people , including me, just tapped thru existing wires to get power to their auxiliary lights and in my experience is not worth doing so especially when you have one of those high end lights like kc daylighters, piaa's or hella. no doubt that they get the appropriate voltage to power the bulb but not enough current that the brightness of a tapped light is just noticeably dim compared to a properly relay wired ones.
Besides, it's just easier to go with the relay system. All you need is a 30A automotive relay (i prefer Bosch, been using them for years to power 4 blinding hood cowl mounted driving lights on one of our previous car) , switch of your choice, 20 gauge wires for the switch, 8 gauge stranded copper wire ( this can also supply +v source for other aux lights that you might want to install later on), 8 gauge copper lug, 30A inline fuse, electrical tape, 12 gauge stranded copper wire (to supply ground to the lights), cold beverage of your choice and 2 hours of your time.
believe me, it made a world of difference when i switched from tapping wires or using the stock wires to making relay harnesses with stranded copper wires for each light. here's a pic of my driving lights that i got from a seller in PA for $19.
Theyre the optilux brand, the cheap hella knockoff from china. Used it already numerous times on dark highways and it lit up the road really far enough to visibly see road signs that looked like small flashlights from the distance. I'm planning to mount 2 5" round fogs behind the grill in the future and would probably do a write-up on it including the relay wiring.