Cavities built into the vehicle chassis have recently emerged as receptacle for hidden antennas. In the automotive sector they potentially replace the roof mounted sharkfin antenna modules. Simulation models for chassis antenna cavities are critical, because they must both accurately predict antenna performance, while staying computationally reasonable. Moreover, if the chassis of electric cars, airplanes and boats are built with carbon fiber reinforced polymer, then a model for the composite laminate is required. In this paper a simple simulation model for chassis antenna cavities is developed. The carbon fiber composite material is modeled as a linear, homogeneous and isotropic conductor, and several cavity geometry details are omitted. Simulation results are in good agreement with measurements in the frequency band at 5.9 GHz for intelligent transport systems.