This paper proposes a new media access control (MAC) protocol for in-vehicle power line communications (PLC). Specifically, the proposed protocol is based on the HomePlug Green PHY (HomePlug GP) which is a modern PLC protocol standard for Smart Grid applications on the Home Area Network [1], however, it has been found that current HomePlug protocols would not cope with the strict timing requirements of in-vehicle communication systems. This paper suggests improvements and modifications to enhance the real-time MAC performance of HomePlug GP in an in-vehicle network through additional priority levels and reduced message length. Simulation results based on the open source network simulator OMNeT++ are provided to show advantages of the proposed protocol.