Use a relay to power your light bars regardless of where you get the switching power from.
If you want to use your fog lights, your brights or a separate switch the method is the same. You will want to use a 12volt automotive relay. If you have it mounted external you will want to make sure it is as weather proof as possible.
You have 2 sides to the relay. The coil, and the switched side. The switched side you have a +12 feed of sufficient size to run your lights to the common and run a wire from the NO lead (Normally open) to power your lights. For the ground on your lights just run to anywhere on the frame. You can easily find other ground locations and add your wire to one of those.
The coil needs to have +12 connected to one side and Neg to the other side. Doesn't matter which side.
If running to a switch the normal way to do it is to put constant +12 to one side of the coil and run the ground to your switch. The switch then can have one side going to a ground near it. You can put a fuse on the coil to prevent it from being fried but I generally just tie the positive side to the same wire that is going to the switched common side. Very short run and doesn't really need a fuse. If you ground the coil it won't harm it.
If running to a power source such as a light you need to ground one side of the coil to the frame and run the +12 side to whatever power source you are powering from. You will want to fuse at the power source to prevent damage to the relay if the wire gets chafed and shorted. I generally use a half amp fast burn fuse but you may find this is not small enough. The coil actually uses very little power. I have never had a wire short out but its not impossible. So you want to have a fuse to prevent your wire from burning up and from blowing out the fuse to the light source you are tied to.