Software applications in the car are gaining in importance as a driver for innovation and value creation for the car manufacturers and their suppliers. These novel software functions, e.g., mobile services or car-to-car enabled applications, are increasingly designed and developed using early prototypes. Building on open innovation literature, this paper goes beyond extant knowledge on prototyping and proposes a novel paradigm of ‘open prototyping’. It assumes that organizations can and should use external input as well as internal input in form of prototypes, as the firms look to advance their technology. Set in the empirical field of the automotive industry, we follow a design-oriented research approach to design, develop and evaluate an open prototyping approach consisting of a toolkit and process. The open prototyping toolkit, HIMEPP, has a component-oriented architecture. Combined with the open prototyping process, it supports the development of diagonal high-fidelity prototypes together with persons from outside the R&D department. The study allows for generalizations to other industries and points to significant managerial as well as academic implications, which can be expected from opening the next step of the innovation process.