ah interesting technique. i must try this too when it gets warmer lol. majority of the time, do u use bare naked flashes (tehehe) or ur umbrella?
I'd say that bare flashes are best for the side where you might get a direct reflection from the flash/umbrella. For 3/4 shots and shots from the front umbrellas are probably the way to go. YMMV, depending on the car and amount of ambient light.