Have a 1987 S-10 with small block 350. Truck runs great. My alternator went out. I replaced it with brand new one. Everything seemed fine. That night I turned on the headlights. The battery was dead when I got home. Bought new battery, replaced alternator with another new one. The truck still runs great but the headlights still drain the battery. Have checked the wiring and see no visible problems. What else could this be??