My driving lights relay is triggered by factory high beam circuit as some others have described here. Most Parts Stores carry a nice little wiring kit that comes with all the wiring, relay, fuse holders, switch, and instructions. Relay with a fused power supply is the only way to go when installing any auxiliary lights.
I added a switch on the dash so if I do not want them to come on when the high beams are on, I can simply deactivate the control circuit (relay)....
Never run the load through a switch without a relay unless it is designed for 30 amps or greater.
Most switches will only handle ~5 amps, anything above that, they go up in smoke and melt....

Wiring diagram:

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