A not working car charger for the Dell Streak can be down to badly made car chargers!
Testing mine, I noticed that my not working charger was being reported by the phone as USB.
So I tried to rectify this, looking at how the phone recognises what it's being charged with.
For the phone to recognize it as a charger, continuity has to exist between the 2 middle (data) pins.
Therefore, I opened my charger, and simply soldered the 2 middle pins together...
Tada! My Dell Streak now reports that it is plugged into an AC source, and it charges!
For 5 minutes 'work', it certainly was worth doing.
Hope this helps you too.