Used to from the "analog world", I wanted to create something similar to what is possible with double exposure technique on film. I got my car back yesteray from service, so I took the opportunity to play a bit. Double exposure is not directly possible on the EOS 300D. So I had to work something out and here is the recipy:
Place the car in a dark garage or wait until dark outside.
Open the shutter in bulb-mode.
Open the bonett and lighten the areas you want to be visible by using a smallisch torch (focused light) as a light brush.
Switch of the torch and close the bonett.
Apply some ambient light for another 2-3 seconds.
Close the shutter.
Sounds simple enough ? Try me, it took me about 1,5hrs to get the result I'm showing here. The biggest problem are unpredictable reflections. Good luck with your own tries and errors.