I have 1990 chevy c2500 3/4 ton heavy duty with manual transmission. I this truck ran fine 3 days ago but now engine won't turn over using the key. I turn over the engine by hand or jumping the starter with a screwdriver.I had the starter tested at autozone also and passed. I replaced both parts of ignition switch. I tested the battery cables with a test light. I had autozone test the battery and passed. This truck had originially automatic transmission but when it was changed it the neutral safety switch was never wired up but its there for the clutch pedal. I don't know if the automatic neutral safety switch was bypassed or finding it.There is a wired plug hanging under the truck were automatic transmission used to be. If somebody can whats that for? I know when the battery is hooked up and when trying to start it with the key it drains the battery. This truck ran fine from previous the owner for over 2 years. Any ideas?