Vehicle sharing systems are the key to sustainable mobility. They need to possess adaptation features to answer the different user needs, and must be automated to avoid intermediaries between users and system. Finally, they must be based on a wide variety of vehicles and on an open ownership model to become a viable alternative to private vehicles. This letter presents the solution devised in the Green Move project to tackle this challenge. In particular, it focuses on the Green Move vehicle on-board units, for which a dedicated middleware is being developed. The middleware allows developers to create applications that can be dynamically loaded/unloaded while preserving the needed safety levels of the vehicle motion functions.