How to Set my iphone 4s to automatically use dock connection?

So here is the problem,

I have an iphone 4s not upgraded to ios 6 software. When i connect it to my car stereo that has built in bluetooth, my phone takes 30-45 seconds to connect usually (which is long… ) but the issue is that when i sit down into my car and use the usb plug and plug it in to the stereo music starts playing but then 30 seconds later when my bluetooth connects it switches my phone to bluetooth mode to play music instead of dock connection. So i want my phone to not switch to bluetooth mode for music but still have my phone connect for calling. The problem is when it switches i have to manually hit a button on my phone to make it use dock connection now and its just a pain… Any advice on how to change a setting on my phone to not switch on me?

I don't know I'm so sorry