Abstract: A system having an integrated control unit for installation in an emergency vehicle, the unit responsive to inputs and operative to output control signals to emergency vehicle electrical apparatus through a pre-fabricated wiring harness. The wiring harness is designed with connectors and an overall configuration requiring only minor alterations to the vehicle structure, allowing easy installation primarily through ducts and openings used by the vehicle manufacturer for conventional wiring installations. The harness is designed to be readily removable for installation in another vehicle of either the same or different model, when the existing vehicle is to be retired from service. The system has numerous features including an opto-isolator and noise filter to provide a reliably detected signal indicative of the state of the vehicle's transmission, the control unit operative in response to modify the emergency vehicle conditions when the vehicle is stopped.