I saw somewhere that Motorola's Vehicle Navigation Dock triggers car mode in the Droid Razr Maxx HD (upon connection to the dock's micro-USB).
What is special about the dock's micro-USB that automatically triggers car mode in the Droid Razr Maxx HD?
I believe that there is an extra pin contact that is sensed by the phone when placed on the car dock, which then activates the car mode. Some car mode apps are also activated by a magnet that is in the car dock. Also,as Yeti35 mentioned, there is a Drive widget that can be manually activated without blue tooth. You basically setup a manual Smart Action. It works well for me when I don't want to use a bluetooth headset.
It's true that earlier versions of the Droid (i.e. OGD and D2, and possibly D3), were triggered into Car Mode by a strategically placed Neodymium magnet in the face of the Motorola Vehicle Navigation Dock (or "Car Dock" and likewise into Desktop Mode with another Neodymium magnet), which lined up with tiny magnetic reed switches (or their solid state counterparts), in the phone's back, so that when the magnet is brought near, the contacts in the reed switch closed making an electrical connection and thereby triggering the desired mode.
However they have moved away from magnets in favor of an electronic "trigger" in the USB port for several reasons. First, it was the problem that many cases and belt pouches had magnetic closures and by simply sliding the phone into the pouch it would pass close enough to the magnet to trigger the undesired Car or Desktop mode and the phone would remain on in the case, thereby draining the battery unbeknownst to the owner. I am one who can attest to that since I used to carry my phone in a Blackberry case which it fit nicely, and I'd often take it out and see it was on in the case. For the longest time I thought it was a button being pushed by the case, but when I discovered the magnet trigger process, I would carefully avoid the magnet on the case when inserting the phone, and the problem went away.
Second, many people desired (and still do), to be able to place the phone into (or on) one of the two dock types without triggering the respective "mode". With magnets as triggers, that was mechanically impossible, so then apps were developed which circumvented that process, adding to the layers of background processes people had to run to make their phones act the way they wanted. This is less desirable since it only adds to the bloat that slows down the phones.
Fast-forward to the current models...
Now instead of the mechanical mechanism of magnets and reed switches, the manufacturers have opted for the use of varying levels of resistance applied across two terminals in the USB port. These specific tiny resistors are electrically conductive but limit the current that passes across these two terminals to a level that is electrically identifiable by the phone's internals, and so when the phone "sees" the 300 ohms for example across pins 2 and 3 for example, it "knows" that you are plugged into the "Car Dock", and when it "sees" 450 ohms, it "knows" you are in the "Desktop Dock". Now, the levels of resistance and pins are likely not the actual values I've used for purposes of explanation but the method and result are the same. You can open the internals of the docs and see the actual resistors there to "trigger" the desired mode.
In the case of the Car Dock on the Droid RAZR/MAXX (but NOT the M/HD/MAXX HD), you can now place the phone into the Dock, but NOT push down the plug, and it will remain in the normal operating mode and NOT trigger the "Dock" mode. In the case of the Desktop Dock, unfortunately that is not the case since the Dock and positioning of the ports are such that the phone is plugged into both the HDMI and USB ports simultaneously. I do not own an M/HD/MAXX HD, so I can't speak to the ability to use the Car or Desktop Docks without being forced into said modes, since from my recall (correct me if I'm wrong), the M/HD/MAXX HD have moved the USB and HDMI ports to the side and so inserting the phone into either Dock involves pressing the phone down onto the connections and thereby triggering the Dock mode.
Also, as mentioned the phone's "Modes" can be triggered by the car's Bluetooth system pairing with the phone upon starting. My Ford F-150 Platinum has MyTouch SYNC, so when I get into my car - even while my phone stays in my pocket, the phone syncs with the car, the car's stereo begins streaming music from my phone, and of course the full feature functionality of MyTouch SYNC are available to me, such as voice navigation and voice commands for phone calls, etc. For at least the Droid RAZR/MAXX, the phone isn't actually going into a particular "mode" so much as it is simply pairing with a Bluetooth appliance, but it "feels" like the two are now one. I often place the phone into the Motorola's Vehicle Navigation Dock in my car, but don't push down the slider that has the USB plug on it so I don't trigger Car Mode as I can't stand the limited operation in that mode and the large buttons are ugly. I suppose that means I'm not a viable candidate for the new Microsoft phone OS either!! LOL!