Abstract: A control and diagnostic system for controlling and diagnosing auxiliary devices in an automobile, the control and diagnostic system comprising: an output terminal for interfacing with and providing control signals to an auxiliary device; an input for receiving control signals from an auxiliary device; a switching device coupled to the output terminal, for alternately providing and preventing the supply of current to the auxiliary device; a fuse coupled to the switching device for limiting the current flowing through the switching device; control circuitry coupled to the input, output and switching device for alternately enabling and disabling each of the input, output and switching device and for providing control signals to the auxiliary device; a switching source for providing control parameters to the control circuitry; and a diagnostic display corresponding to an element that is to be diagnosed, for displaying the status of the element.