If you look to the right of this page, at the messages in the "More Like This" section, you will see that quite a few people have had problems with car chargers, but generally it's been using non-Sony Ericsson chargers, but others have had no problem at all using generic equipment. There does not seem to be any obvious reason why some people are experiencing problems.
I assume that the 12v socket in your car is working correctly and providing power to the chargers...?
