Aftermarket wiring

[domenic217]

Just wanted to get a feel as ive never done something like this before and want to get an idea on how it’ll work, 

im looking into getting a a light bar for my 16’ Sierra and have existing stock fog lights on the truck would there be a way I can tap the light bar into the fog light switch so the light bar is only on when I turn the fog lights on. 
 

also just wondering I have a couple accessories (train horn, smaller lights that’ll get there own switch, etc.) but is there a way I can hook them up where they will lose power when the keys off. My guess is the a fuse box that is set up to shut off when the keys off, but how would I tap into it. 
 

Thanks in advance appreciate the responses when they come in. 

[superdave160]

If I were going to do what you are suggesting with the light bar, I would purchase a light bar wiring kit (if the light bar didn't come with one).  A light bar wiring kit usually comes with a harness to provide power to the lights, a relay to control the power, and a switch so that the driver can turn them on and off.  The switch is usually wired to the positive coil of the relay so that when the switch is ON, power is applied to the coil to energize the relay.  I would cut the switch off and run a wire from the power wire for the fog lights to the end of the wire that runs to the positive coil on the relay.  The negative coil of the relay should be wired directly to ground.  The power wire for the light bar should be connected to the battery, then go through the relay, ending up at the light bar.  This way, when the fog lights are on, power will energize the relay and also bring on the light bar.

[Dresari702]

What Dave said.    The factory wire powering the fog lights is likely too small to power both the fog lights AND the light bar (directly).   The fog lights will power the coil of the relay, which is a small energy draw.   This will act like a switch to close the relay contacts and turn on the light bar.