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.
- The Great Szalam
(The 'Great' stands for 'Not So Great, in fact, Extremely Humble')
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