Ecm wiring dirgram

What is your question?

98 silverado 5.7 without anything on in the truck it charges at 13.5 volts when you turn on anything like headlights or heater it drops to 12 sometimes less alternator an batter have been replaced does anyone know what wire controls the voltage from the ecm?

How long has your 1998 Chevrolet C1500 had this problem?

Truck been parked over a year