Sadly I don't have any external flashes, going to invest in some when I get the 7d/5d. What I have been doing with black cars is either slight hdr, or 2 exposures one exposed for the car, one for everything else and mask the well exposed car shot into the background one (if that makes sense). I have had much better luck with white cause, I learned that shooting them in shade is a must though.
This is with a white s5, half in the shade and half out so it kind of shows when its in the sun it gets blown ( I even edited it to bring out more detail ) But in the shade you can see the lines quite good and you get great reflections. Sorry if that doesn't make sense haha.
Winking :: S5 by
Trevor Thompson(NEW FLICKR), on Flickr