Yes there are several ways to do this (in order of preference):
0) If you have or are willing to, buy a bluetooth a2dp receiver or a car radio that comes equipped with one. Pair your Archos to it. Profit.
1) If your car has AUX in, then just plug Archos headphones out to Aux in. Profit.
2) If you car has a cassette player, then use a cassette adapter and plug it into your Archos. Profit.
3) If you don't have the Car mount, buy a cheap stereo FM transmitter and plug that in. Don't worry about the Archos built-in one. Profit.
4) If you have a Car mount, try the Archos FM transmitter. You may profit.
Except that the Car mount won't give you the same sound quality then the AUX IN. The sound quality with the Car Mount passing through the FM receiver is so so compare to the AUX in.
Unless you live in the middle of nowhere using an FM xmitter just doesn't work in the US, or is such a pain that you are frustrated all the time ... AUX in unless you don't have one, if you're going to change out the radio, it seems buying one with a USB input is far, far better then you can just put the music on a USB and you're all set with no charging BS.