Droid 4 OEM motorola car dock: how big is the ball?

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http://forum.xda-developers.com – It looks like the droid4 oem moto car dock attaches to the base with a ball joint. Can someone who has one disassemble it (should just be able to unscrew the tightening ring) and measure the diameter of the ball? Also, can you post some pictures of the dock from the back? It's possible that the dock clips onto the ball somehow, rather than having that part built-in. I'd like to know that if possible. I'd like to put together a permanent installation dock for my wife's d4 for christmas, but I can't find any pictures of the back to see how this hooks up. So I have no idea what adapter to b (HowTos)