I'm not sure if this is possible.

I'm currently creating a Car booking system.

what i want i want to be able to do is. When the car is selected display a calander with the days the car is already booked highlighted in some way.

Problem! Idont have a clue about this one.

Can anyone else help. Provide code, what ever. Any code used i will give full acknowledgement to

I'm guessing no one else has much of an idea either then.

What you're looking for is a bit more than just a few lines of code. You'll probably need to write something yourself if you can't find a script that does this sort of thing already...
Firstly, you'll need to store the days the car is booked somewhere - most likely in a database. The, when displaying the calenday, iterate through each day and chack the DB for an entry on that day. If there is one, set the highlight colour.

I use a free calendar component. I'm sure you can do your script based on this page example:
http://www.mylittletools.net/scripts/en/mlc/methods.asp

Let me know if you need my personal example.