Electric net
The best way of catching flies attracted to a car, is to use an electric net. These nets consist of an array of very fine wires connected to a high voltage generator. Flies collide with the grid and are either killed and stunned. They then fall onto a sticky trap, or some sort of retaining tray, placed at the bottom of the grid.
Electric nets are specialised devices requiring high levels of maintenance, and they are generally only used by research scientists. However, government entomologists in Zimbabwe and Botswana have mounted electric nets on the back of vehicles to survey for G. morsitans.