If they're only dusty, a small soft-bristled paint brush usually does the job. For heavier dirt, a toothbrush and dish detergent works well, like others have said. If you are going to repaint the vehicle, you need to ensure you rinse the detergent off completely before doing so. There are also several types of "goo remover" commercially available which are excellent for removing the glue residue left behind when old decals are removed.

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"The future's uncertain and the end is always near." - The Doors "Roadhouse Blues".