Take all the pins out of the plug, push on all the female pins with a small screwdriver, and reinstall the pins. You will need an extraction tool to get the pins out. Check all your connections where the other end of all the wires from the plug terminate. I have pulled on them and had them come right out of their terminals.
This battery connected backwards could be a faulty input to the control card. The card has to have a good positive and negative. On the ev100, negative is the brown wire in that 14 terminal plug. Not sure about the EVT100 which is what is in this truck.