Unfortunately, the stock wiring isn't big enough to handle most 40" light bars. The wiring (and fuse) are rated for 15 amps. Most 40" LED bars are going to use approximately 20 amps (19ish, usually). Please don't be that person who says "well I won't have any issues" and just hooks it up to the factory wiring. Trust me, stretching electrical systems beyond their capacity is nothing to play around with. And a stretch like that (33% above the designed capacity) is just asking for a major meltdown. The last thing you'd want is a burned up rig in the middle of the desert.
So you'd need to run larger wiring to the roof no matter what, unless you get your hands on a smaller light bar that uses less than 15 amps (maybe a 30" or something?). 40" really is the ideal size for our rigs, and if you can get your hands on a set of those Offroad Gorilla mounting "devil horns" mounting brackets, it's a perfect setup. If you still want to run it off of your factory switch (to keep a clean factory look inside the rig, etc.), you can just install a relay along the factory circuit that acts as the "on" switch for the light bar when you engage the factory switch.
The quick way to calculate the amperage draw of a light bar is to divide the wattage by the operating voltage. For example - a 40" light bar with 80 LED bulbs that are 3 watts each would be 240 watts total (this is a very common configuration for a 40" light bar). 240 watts / 13.4ish volts (typical operating voltage of a stock alternator) is 17.9 amps. If your alternator is putting out less (or you're running other accessories that are dropping the voltage), or you shut the car down and the light bar is running straight off of the battery, it would be more like 19.7 amps (240 watts / 12.2 volts). So if you keep your eyes open for a light bar that will draw less than 15 amps at 12 volts, you'd be good to simply wire it right to the factory circuit. Otherwise, you will need to run larger wiring to the roof.
