The FM ttransmitters are not 100% reliable in my experience. These are weak FM transmitters that transmit the audio from your portable device to your car antennae and then you tune your car radio to the transmitting frequency. I tried a few in my car and becuase the antennae in my car is over the back left wheel, the reception was quite poor. If I moved the transmitter closer to the antennae, it got better but hardly ideal. If the antennae is located towards the front of the car, these should work better.
Also some of these have only 4 transmitting frequencies so you will need to make sure that there's no local stations transmitting in and around the same frequency.
The most reliable option is to run a 3.5mm stereo mini-jack cable from the back of the stereo out through the glove compartment. Whenever you want to get use the Archos, open the glove compartment and plug it in. You will need to check the manual of your car stereo to see if it has an aux input on the rear.
I hope this helps.