I have to credit techitrucker at chargeforum.com for coming up with this solution which I have rewritten as an outline:
1. Disable auto launch in dock settings.
2. Get AutomateIt from the market. The free version will do the initial switching to whatever car home you want to use, but if you want the home key to go back to your car home from any app instead of the launcher home then you will need the Pro version (1.50) and an alternate launcher (I use go launcher ex but there are others).
3. In AutomateIt create a new rule. The trigger will be dock state set for docked to car. The action will be start application set for whatever car home app you want to use (I am liking carhome ultra). If you use the pro version you can setup a composite action that will also adjust volumes; enable/disable gps, wifi, bluetooth, etc; or anything else you want to do.
4. If you want the home key to return to your car home app, using AutomateIt pro create another new rule. The trigger will be a composite AND trigger with dock state set for docked to car AND application activated set for your alternate launcher. This won't work if you don't use an alternate launcher because the native launcher won't show up as an application to select. Action will be start application set for your car home app. This rule will cause the launcher home to appear momentarily when the home key is pressed, but your car home will return in about a second. An alternative not using AutomateIt pro would be to long press the home key and select your car home app from the list of recent apps.
Again, thanks go to techitrucker for this procedure. I hope it helps someone. I suspect the native behavior will never change because Verizon likes to push vznav for the extra income.