A vehicle harness structure and an additional connection member are provided, according to which part numbers of wire harnesses are reduced and superfluous attachment in the wire harness is avoided. The vehicle harness structure includes a basic harness for interconnecting a plurality of main devices to be mounted in common on target vehicles, an additional connection member having one end connected in a branched manner to a communication line or a signal line of the basic harness and another end connected to at least one auxiliary device to be optionally post-mounted on the target vehicles, and a control function section provided in the additional connection member to control the operation of the auxiliary device.