[Context and motivation] The use of conceptual models in automotive requirements engineering is impaired due to the lack of appropriate modelling guidelines. [Question/problem] The goal of this paper is to propose a requirements reference model that serves as the basis for defining such guidelines. [Principal ideas/results] The reference model distinguishes three abstraction layers and three content categories for requirements models. [Contribution] The reference model has been successfully applied in the REMsES project to support the development of a model-based requirements engineering approach for the automotive domain.