I have an 82 6.2. When I turn the key on, the dash lights light up, including the glow plug light. Lately, (randomly) when I turn the ignition on the dash lights are lit, but the glow plug light is not. That is when the truck won't start. It acts like the battery is dead. It will turn over, but get weaker and weaker like no battery. However, the batteries test fine. When the glow plug light is on when I turn the ignition, the truck starts fine. I'm thinking some sort of electrical? It's like the power isn't getting from battery to glow plug, or something is draining the electrical/glow plug. Help. Thanks.