I didn't want to post this on the board, but we deal with this all day everyday, the only possible way to get your deposit back is to lie to the dealer, tell them ANYTHING but you've "changed your mind", I know this is horrid but it'll have to be a reason like a death in the family or something, or you lost your job and can't make the payments anymore, still they don't have to give the deposit back.

We've had this happen so many times, but it costs us money to advertise, the finance company charge us a fee if people back out and more importantly, when I take a deposit the car comes off sale so we're often turning people away that will actually want to buy the car.

You're best off chatting with the salesman as they may well let you have it back if you're nice to them or they'll transfer your deposit to another car

I know it's probably not the answer you were after but the conditions of deposit would have been explained to you when you signed the finance forms