using iphone with cars audio system

My toyota 4runner has a eclipse tomtom audio system with an ipod connecter which allows me to dock my iphone and play my music library (or pandora) using the cars audio. The audio system also has an annoying bluetooth hands free function that sucks because it has a terrible mic and most people on the other end complain of a terrible connection. I usually use the head phones that came with the phone for the hands free but the trouble is that if the headphones are connected the music wants to come out of the head phones too and not the car's speakers. Is there a way I can set that without disconnecting the headphone jack - as that is very difficult for me while I'm driving.

I have an iphone 4s

Thanks.