Motorola has an app in the market that when used with there visor mount device called the Roadmate or Roadster might be able to do what you described,but don't quote me on that. I have one of there visor mount BT devices and I can here it good with the windows rolled down. And people can hear me with no problem.
I haven't tried to see if it will work for nav. but I have an app that came with the BACKsidr ROM that speaks everything back(And I do mean everything) I'll try it out on the nav. and see if it works. It can be found in the market by searching for accessibility application. It's called TalkBack. You might just have to wear the headset in one ear is all.
Edit: The stock google nav. talks back fine with the supplied headset. I had to turn the volume down. If nothing else you could just do that.
Talkback will read mms,e-mail,and Every little thing you do to your phone. It will allow you to keep your eyes on the road more though. And if you just have to type anything into your phone while driving it will speakout the letters as you move across the key board,and then type the letter that your finger was on when you lift it off the screen.
It will also allow you to navigate around your phone without having to take you eyes of the road.
I have my phone hooked into my speakers of my car thru the stereo. That works really good except for when I was getting calls and notifications,then it's way to loud.
I found an app that will turn down these volumes while hooked to the headset jack, and they go back to normal settings when its taken out. It is called "Volume Control". You can find it in the market.The icon has a white background with two sliders and a headset plug placed diagonally across.
