many car power adapters don't have the amperage to both charge the phone and provide power for gps/3g simultaneous use...ie: power is being consumed as fast as it flows in.
were you using nav? did you find that the battery was at the same level when you left the car as when you got in (ie: you used the phone, but burned no battery)?
if so, you need a more powerful charger. i had the same problem and bought a 1 amp version. ran gps/nav/email data feeds/made some calls and did gain some charge. i think even more amperage would be ideal, but that's all i could find. my previous charger was ~0.6A.
