The increasing number of electronic and electromechanical systems in cars requires electromagnetic compatible harness designs. The target is, to evaluate the consequences of different cabling-layouts on electronic systems in a very early design phase. To realize this practice oriented EMC simulations of wire harnesses one has to follow different steps. These very steps are described in the presentation.We propose a preprocessing methodology for the CEM that can be performed in an efficient manner by automatization through software tools. In the first step, it is shown how the car body and harness data is directly extracted from a CAD system. An automatized partitioning of the harness and its environment is described. CAD functions and special algorithms are used, that allow for judgement of the harness structure as well as for frequency dependent user control of the relevant spatial range between the harness and surrounding car body parts.It is shown how the first step delivers appropriate geometric models of the harness. As a next step, the automatic processing of the geometric models and their preparation for the two- and three-dimensional numerical field calculation of electromagnetic fields is shown. Finally, the extraction of system models for an abstract level system simulation of the car network is shown.