You may want to try the following link: http://androidforums.com/htc-desire/177579-froyo-car-panel-desire-htc-navigation-panel.html
The first post should have links to three different "Car Home" applications: Google Car Home, Google Car Launcher, and HTC Navigation Panel. You will need to either go to this site on your phone and click the links or download the .apk and transfer it to your phone. After reading a little more down the page it seems if you choose the Google one then you need to install both not just one of them.
I haven't tried these on my Hero yet (I will tonight after work) but in my experience trying to install apks either works or it doesn't - so give these a shot and see if any of them work. If anyone has had different experience installing non-market apk's please sound off in the comments.
I also just found this page: http://geekfor.me/hero/downloads/fresh21removedapks/
These are apps that are removed from the fresh ROM 2.1 for the Hero. As you can see from the list the HTC Navigation Panel is in there. However I don't know if you need to be on fresh ROM for these to work.
All that said, Byran's Car Dashboard look like a pretty cool replacement if you don't need the official Google or HTC car app.