A tri-state electronic vehicle system control circuit comprises a plurality of 3-position switches, each of which is coupled to a dedicated pin on a wiring harness for coupling multiple portions of a combination vehicle such as a tractor-trailer combination. Each switch controls a device (e.g., brake lights, turn signal, etc.) and a data link that can be selectively toggled in a manner that permits data transmission without perceptibly affecting device function. Alternatively, two devices controlled by a given switch can be enabled by a single pin on the wiring harness. The tri-state switching circuitry thus doubles the capacity of the wiring harness to which it is applied.