I think it would be funny to wrap up a $1 in a small box with a little note that says $$$ for your car (open first). She will be dissapointed (it may be funny to see her reaction...I would have just thanked my parents and fumed inside) until she figures out that there is a check taped to the back for the real amount so it isn't obvious...or put a little cash in each of her other presents for the real amount.
You can also get fake money for the real amount and a little note that says $$$ for you car...and on the back of each in pen that you can't see through to the front, put "Redeem for the real thing"! It depends on her personality if she can handle the joke fine.
I like the old car idea too, but put the money in the trunk and maybe take a copy of their (or yours) car title and white out the correct name and put her information so it looks "legal" with a stamp "PAID IN FULL" on it and wrap that for her to open first! On the back can be "Just kidding" and a map to find the real money in a car kit.
Oh, sorry...another idea. We have a hard time getting our dad with gift, so we bought him a ping pong table and left it in the truck in the garage and then only wrapped the balls and paddles and made him play "20 question" to guess his gift. It was hilarious that he couldn't guess the gift off of the package. You could do something similar with the money or the old car or both...
Good luck and have fun!