[RNRea]

I would suspect that there is a problem with either the ground connection or either battery cable connection at the battery. I had a vehicle that acted like a flat battery (lights would work, but a little dim). When you tried to start it everything would die. Removing the battery cables from the battery and cleaning everything fixed the problem. Other areas are the battery ground connection to chassis / engine, the cable ends themselves (especially if they are the type that clamp to the battery cable) and corrosion on any of the wiring connections.