There is a degree of knack to it, but the set up for most photographers is usually the same, depending upon equipment limitations (such as max synch speed).
The idea is to remove as much as possible of the ambient light from the equation, and rely on the flashgun being the principal method of illumination; this usually means exposures of 1/500th sec at F11 in daylight, with the flash exposure compensation being set to zero or +1 stop.
Some older cameras (such as the DCS520 and the 540EZ required more compensation, and they only synched at 1/250th (from memory)). If shooting on a wide (16-35mm) up close to the side of the car, usually manually focusing at about 1 metre should give sufficient depth of field to cover the target.
Typical of a car shot, this was done on Canon EOS1D using an old 20-35mmF2.8L, with a Canon 550EX Flash, Quantum Turbo Battery manually focused at 1 metre, and exposed at 1/250 @ F11.
Pictured are Kate Winslett and Jamie Threppleton, her former husband, just days before the split.As usual for celebrity events, the cars were heavily tinted, and raced through the press pack (of which there were about 16 snappers and half a dozen reporters).
If shooting on a long lens through the front windscreen things can be a bit more hit and miss, as you are generally further away and the flash needs a lot more power (hence most snappers use Quantum or Digital Camera Batteries,) and reflections of the sky can be a problem; the advantage is that the front screens are seldom heavily tinted.
Whilst everyone else got shots of his ears, this made the front page of The Sun and a big spread in the Daily Mirror.Shot on a DCS520, 70-200 F2.8L at 1/250th Sec @F9 at 200ISO Flash at +1 Stop.
One of a sequence of 4 frames. One Shot AF Mode, and manually re-focused. More pictures of Billie Piper and Chris Evans from this job are in the galleries on the Press-Photos.com website.
Prison Van shots tend to be even more hit and miss, the windows are very high up and very heavily tinted. The prisoner is generally seated at a level below the window, there are also between 4 and 8 windows on each side of the prison transport each looking into a small holding cell, so the odds on getting the right window is remote.
Generally snappers will agree to cover different sides of the truck and share the results. I have only (to date) had one successful Prison Van shot, and that was of the wrong prisoner.
Unfortunately Sods Law usually comes into play, when you’re on a job on your own. Exposure for prison vans varies depending upon the camera/flashgun combination I generally shoot at 200ISO with the Flashgun on half power and 1/250 sec at F8 or F11.
The same settings can be used on any vehicle that has Limo tint , which strangely enough includes most Limousines.