Wired Headset and Cars

I am sure this is fairly simple but, i can't for the life of me figure it out.

I connect my HTC to my car using USB power and a standard 3.5mm cable (stereo, with two rings on the jack) if possible i would like to be able to divert the callers voice to my speakers through the 3.5mm jack and my voice to be picked up by the mobile mic.

The bit that i am having issue with is getting the phone to automatically push the audio to jack when plugged in but through the phone speaker/ear speaker when not plugged in.

Even better would be to set it to go throug the ear phone jack when docked in the car dock (offical HTC)

Let the flaming commence.