Another method is to sprinkle sea salt on the floor of the car. This will help to absorb negativity, but make sure to clean it out often. You can also wipe the surfaces with salt water. Salt water is believed to be able to ward off negative energy.
In Malaysia, we use pomelo leaves and kaffir lime leaves (daun limau purut) to clear out negative energy. Incense and kemunyan are also used for cleansing. However one needs to be careful as it may leave a strong smell.