My n900 auto connects to nokia car kit via bluetooth.
First 2 weeks no problems.
Know when it's connected it's keeping the car kit activated so i can not listen to carstereo/cdplayer, it's mutes the carstereo all the time.
After disconnecting the n900 sound is comming out of the carstereo speakers.
With an other phone no problems.

Notice: Instead of a car picto i see a headphone picto on the n900.
I deleted the devices and re-paired with the carkit but still the same problems.

Any suggestions?

Are you on the latest firmware? I had this problem but the January update resolved it for me.