I'm designing a used car sales website and the client would like for customers looking to sell their car to have the ability to enter their car registration number plate, and for the site to pull back the make and model automatically.

There are many sites that do this (http://www.halfords.com / http://www.gocompare.com / http://www.confused.com / etc)

But I'm drawing a blank with several hours of googling providing nothing of use.

Can anyone provide a clue?