In the app drawer, navigate to the stock Music app by HTC ? the one with the red musical note.
Tap*Menu*>*Add app shortcut.
Select your desired app from the list, but keep in mind that you?re limited to eight shortcuts in the music app. (Interestingly enough, the app you select doesn?t necessarily have to be a music-related app; you can select Gmail, for example.)
Next, open the Car app.
Swipe from right to left, until you reach ?Music apps.? (You should only have to swipe twice.)
Select whichever app you want.
Exit out of that app, and exit out of car mode.
That?s it! The next time you enter car mode, the app that you selected above will appear as the fourth icon in car mode.yt?