Give the light trail technique an extra twist by shooting from a moving car. But first point, safety. If you want to capture light trails from a moving car get a friend, family member or a fellow photographer to do the driving so you can concentrate on taking the photographs from the passenger or backseat. It also means it is easy to review images taken and make any changes without having to pull over.
Before you set off make sure the inside and outside of the windscreen is very clean because smears can ruin your final image. For the same reason you may want to avoid rainy evenings unless you want smeary colours and your wipers going.
You need to fix the camera, with a wide-angle lens on a tripod into the passenger seat well or back seat of the car. Whatever you do, make sure the tripod is fixed securely before setting off. This is not to stop vibration - which is inevitable - but to stop the camera smashing itself as the tripod topples over as you drive along. You can use a full-sized tripod but something like a Gorillapod is also worth considering. You can even use a beanbag if your car has a sizeable parcel shelf. Whichever approach you take is obviously down to the car itself as well as the sort of shots you want to take.
For example, you can have the camera positioned so you can see all of the windscreen through the viewfinder and don't worry if you get too much of the roof or dashboard in as you can crop these out later, but it is good to give the light trails something to contrast against.
Focus, Composition And Exposure
Focus on the distance - you want the lights sharp ideally. Check the composition and set an aperture that will give you a good depth-of-field to render the dashboard as sharp as possible as well as allow a lengthy shutter speed. ISO100 is best and will give a slow shutter speed that will give you a 10-30 second exposure to capture the light trails perfectly.
You will have to do some tests to see what suits the lighting conditions and the effect you are after. In a brightly lit town soon after sunset, a 20 second exposure is fine but you might need much longer exposure on a dark dual carriageway. Use a remote trigger, cable release or self-timer to fire the shutter. If you want to use long shutter speeds, use the B setting and, ideally you will have a release that will let you hold the shutter open.
When And Where
For this technique, twilight is a good time and you need a location with a variety of light sources. A mixture of traffic lights, shop lights and street lights will give you different colours and streaks at a variety of levels. While lights on the motorway will give you long, continuous streaks of light, going to a variety of lit places and taking many, many images is the key to this fun technique.
For white balance, use AWB. Colours are going to be all over the place anyway, so start with AWB and try incandescent if you want less warm images.