I purchased a car dock for my droid razr. When it was new it connected to my phone and gave a "car dock" screen. However this is no longer working for my phone. I just use the mount to hold the phone, I have not connected the charger or anything. It worked for 2 weeks perfectly, but now nothing...
I have tried a reset of the phone, as well as clearing data on the dock program under applications. I have an HD dock that does still work with the phone.
Since your phone still works on your HD dock, it has to be the car dock.
Inside the car dock is a small circuit board. That board or the wires from that board to the micro-USB port that slides into the phone could be bad.
I don't have the packaging my car dock came in, but there should be some kind of warranty (maybe 30, 60 or 90-day?).