Maybe there is a Car widget?
The Car app may be "hidden" on your phone. ES File Explorer's app manager is able to show hidden apps and to create a shortcut of hidden apps on the home screen. On its App Manager you'd look at 'System Apps' for the Car app.
Note, the Car app may be hidden because HTC wants you to use their car dock.
On the car dock's microUSB connector the pins 4&5 are jumped with an electrical resistance (about 10kOhm).
This will trigger the Car app.
Harry