I want to put two small light bars under the rear bumper of my truck.. I have a 2015 1500 double cab.. I want to wire them into my bed light switch in the cab.. I can do it just need some advice as in how can I find that wire and what gauge wire I should use and if I need any fuses or relays.. not very educated on how to draw up electrical work but I can do it when im told how

 

Take a look at the upfitter manuals for wire routing diagrams
https://www.gmupfitter.com/pages/body-builder-manuals

First you need to know how much power the light bar consumes.