Cooperative driving systems enable vehicles to adapt their motion to the surrounding traffic situation by utilizing information communicated by other vehicles and infrastructure in the vicinity. How should these systems be designed and integrated into the modern automobile? What are the needed functions, key architectural elements and their relationships? We created a reference architecture that systematically answers these questions and validated it in real world usage scenarios. Key findings concern required services and enabling them via the architecture. We present the reference architecture and discuss how it can influence the design and implementation of such features in automotive systems.