As I said in the basics forum, where you also posted, you may be able to use motion tracking to track a black solid onto the license plate. This page from the AE manual has information about motion tracking (and includes links to tutorials and other things such as the Mocha tracker that ships with AE CS4.)
You may end up having to do some rotoscoping.
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Re: Masking by Sean Doyle on Jan 2, 2010 at 5:22:19 am
Hi, well, sometimes motion tracking can't be so accurate but it does work very quickly. If you want this alternative method that takes longer but is more accurate then keep reading on. Now I'm not sure if im going to tell you this right because I have only experimented with it once and barely remember the steps I did. So, create a Black Solid (Layer>New>Solid) and make it comp size, then create a mask with the Pen tool over the license plate on the first frame by selecting the Black Solid layer in the timeline and pressing "M" and then set down the stopwatch on Mask Shape (or Mask Path if your using an earlier version of AE) then you animate the mask with the Pen Tool throughout the whole Composition. Now I think that there are some other extra steps to doing this but what you have now should be good enough.
If this didn't work, I'm truly sorry for wasting your time, but this is all I can remember, Hope it worked!