closely eyeball the power cables from the motor to the controller, especially where they run under the battery, (pull out battery and clean that area, and allow to dry completely,) and replace any cables that the insulation is cracked or have been sitting in battery acid produced corrosion.
also be SURE the backup alarm (and all other after market accessories like a CB radio) are/is correctly wired in to both positive and a good negative that is for _sure_ a battery negative, and not just a wire that seems to have a negative value when tested.