Its the switch on the master cylinder. It basically is a pressure switch that the cruise control uses to disengage when braking. What happens is that it may leak, and the switch is fed power 24/7. If it leaks, and then shorts, it lights the brake fluid on fire.
So, it starts at the brake master cylinder and it spreads quickly from there. Its also been known to happen with trucks that have been turned off and parked for a while. It can happen at any time. It is very wise to take it to the dealership or put a fuse in like I described above. That way, it blows the fuse as soon as it shorts and it doesn't burn.
BTW, the positive wire should be red or red/white. And, if you don't use cruise or it doesn't work, you can just unplug it.