Ignition switch possibly? I damn near lost mine due to that. I was under the hood when mine decided to do it's thing. My truck had been sitting for a couple of weeks, went out to work on the fuel filter conversion I believe - totally under the hood, didn't start it before working under the hood. Just opened the d-side door, opened the hood, and went to work. At one point I looked up and through the brand new windshield I had just had installed and thought, may, that looks like smoke - it didn't only look like smoke, it WAS smoke. I got the batteries unhooked pronto and nothing else happened, well other than I had to change out the switch and vent the cab as it wreaked of burnt wiring. I also unloaded much of the switch to hopefully keep it from happening. Funny how it sat all that time and decided to do it when I was standing there though, otherwise, I could have been an hour away at work and just come home to the pile of ash!