Abstract: An electrical system for a vehicle is provided. The electrical system includes a control unit configured to control operation of one or more vehicle systems. The electrical system also includes a multiplexed data bus communicatively coupled to the control unit, and configured to carry multiplexed signals to and from the control unit. The electrical system also includes a relay control box. The relay control box has one or more relays, and a controller. The controller is communicatively coupled to the multiplexed data bus, and is configured to decode the multiplexed control signals from the control unit to generate one or more decoded signals, and operate the one or more relays according to the decoded signal. A method of retrofitting a motor vehicle relay control box assembly is also provided.