If it is a third party dock, and you're handy, you may be able to modify the dock to enable car-dock mode automatically. Dock mode is enabled using a small magnet placed at the right location in the docking station, if you play around with the back of your Droid you may be able to figure out where the sensor is (on my D2G it seems to be around the Google logo on the back, but with my test magnet it's not very consistent - the magnet may be too big or too weak to be reliable). Note that Motorola phones have two dock modes, a multimedia dock and a car dock. Usually the magnet will trigger one near the bottom of the phone and the other near the top.
Edit: After a little experimentation the problem is that I was using a canning magnet (a magnet attached to a plastic stick) which makes it difficult to flip over, I was holding the N side of the magnet to the back and it needed the S side (or vice versa). The spot is actually between the Google and Verizon logos over the top of the battery door. The multimedia station spot is on the side closest to the USB port, the car dock spot is on the side close to the volume/camera buttons.