How old is the car (new I guess, but how long have you had it). You have the right to reject the vehicle only when you pick it up from the dealer when the car is new. The best you can hope for now, if you want to get out of the car is for a 'buy back' from Fiat or a dealer 'buy back' where either fiat or the dealer buy the car back as if it were a part ex and the car had no problems... you'd definatly loose out... failing that you can always negociate a part ex with your current car on a new car (fiat always want to sell another car) make sure you get above asking price on your current car... i.e. by a good 3-4%, you should also be able to knock up to 10% off the new car depending on the list price. (a dealer usually makes <12% of the list price on a new car, the cheaper the car the less money at stake, the less likely they are to want to make the deal).
When you pick up the car make sure that you arrange the pick up during the day on a sunny day (difficult to predict I know) if it's raining, ask to pick the vehicle up the next day... make sure that there are no chips in the paint work and that the paint is evenly distributed over the car, if you suspect that any part of the car has been touched up or will need re-spraying do not accept the car (incedently I don't know how this will affect your cash deposite, however if your deposite is your old car, you shouldn't have a problem, so long as you haven't signed the V5 over to the dealer). Also keep in mind that the paint on a new car fades quite rapidly from the time it is sprayed in the factory until it gets to you, so when parts need spraying, it is often dificult to match the colour of the car using the colour codes from the manufacturer.
Things also to check on a brand new car are:
- No transport fixings in the suspention/breaks (bits of plastic that stop the car moving too much during transit)
- Spare wheel (especially if you asked for space-saver or alloy spares)
- Radio Codes are keyed in
- Red/Master keys
- Mats if you ordered them
Also, if you want to be extra sure, you may wish to take the dealer on a test drive before you take your pride and joy home... however this may constitute your acceptance of the vehicle... if you are at all unsure about the car ask the dealer, and get him/her to write down any defects that they promice to fix, and have the vehicle booked in then and there, if possible leave the vehicle with the dealer until the problems have been resolved... thus you have not accepted the car...
hope this helps