This paper presents an object-oriented method customized for a tool-assisted development of car software components. Tough market conditions motivate smart software development. ASCET SD is a tool to generate target code from graphic specifications, avoiding costly programming in C. But ASCET lacks guidelines on what to do, how to do it, in what order, like a fully equipped kitchen without a cooking book. Plans to employ the tool for BMW vehicle software sparked off demand for an adequate, object-oriented real-time methodology. We show how to scan the methodology market in order to adopt an already existing method for this purpose. The result of the adaptation of a chosen method to ASCET SD is a pragmatic version of ROOM, which we call BROOM. We present a modeling guidebook that includes process recommendations not only for the automotive sector, but for real-time software development in general. The method suggests to produce early prototypes that are validated and refined to completion. BROOM offers phase-independent, harmonic guidelines. Product requirements, in form of scenarios, are transformed through several activities into operational models. BROOM takes advantage of ASCET's rich experimentation- and code generation features. These allow to validate emerging models on button press. The factual development of a simplified heating/cooling system at BMW serves as a running example throughout the paper.