2005 GMC 3500 has V8. Everything was fine...except the voltage gage read pretty high...16v or so. Got to where I was headed, stayed an hour and then left. Everything fine again. Suddenly on the ride home the volt gage went to zero. At this point I no longer had high beams and could not turn on the dome light or adjust the dash light dimmer. Got home...no backup lights. Shut the truck off. Headlights and dash lighting won’t go off. Disconnect battery and everything goes out. Wait a minute, reattached battery. Big spark. Truck starts but no headlights, tails, parking lights, etc. I do have wipers, fan blower, turn signals and brake lights. No dash lights. All gages work but volt gage reads zero.