My experience with car financing is yet unique that I have been able to cancel it before car delivery loosing only processing fee.
In Feb 05 I booked through Meezan bank (Islamic) and paid installments till August 05. My car was to be delivered on 22nd Aug. But then I had to leave for UK and I decided not to take the delivery.
This is one of the cases where Islamic banks are different from other banks. Their contract allow you to cancel it at any time and I did that.
As far interest rate is concerned the islamic bank calculates rental and that also uses the prevaling interest rate which creates confusion. Rental also does not change what is agreed in the beginning as people experience that in conventional bank with increase in discount rate the interest rates which were not fixed were increased as well and people have to suffer .