I had the same problem with my Trooper and it was the alternator. The system cuts power to none essential stuff thats why heaters, a/cs, and electric seats quit. The alternator that comes on the truck is a 90 amp alternator. The one that Autozone sold me was a 75 amp alternator ..no one showed a 90 amp alternator for that application so when you asked for an alternator, they sold you a 75 amp. When I put a 90 amp alternator back on from salvage yard it worked fine. (I couldn't get mine rebuilt because they had already disposed of my core)