Airplay in your car

I have set up an Airport Express in my car so I can Airplay from my Iphone to the car stereo. My car has an aux jack I plug into from the airport express and it works great from Ipad, Iphone and Ipod Touch. BUT, since I have no internet connected to my car wireless network I can't use the wi-fi to airplay and still have an internet connection for my Iphone or Ipad 3g. Does anyone know of a way to still use the 3g for everthing else and still use wi-fi to airplay. By the way it is awsome to use airplay in the car but I would still like to have the 3g internet connection at the same time.