The amount of electrical loads and the complexity of the electric system in a vehicle is increasing at the same time as the allowed development time is getting shorter. Possibilities to predict the electromagnetic behaviour is therefore of great importance. In this paper, a method for determining the parasitic components affecting crosstalk in a cable harness is proposed. The method suggests that a more complex ground structure should for calculation purposes be replaced by equivalent ground planes. Calculated parameter values are compared to values gained from simulation in a circuit model for investigation of crosstalk. It is shown that also an approximation of the parasitic components will give a good comprehension of the crosstalk effects in the cable harness.