and in March this year finished a 3-year loan I took from them. My car still has 1.5years to go...

Yet I don't understand what you are talking about with regards to your loan now being QR5,000 more...

If you take your loan over a certain period, with fixed installments, how does it grow to QR5000 more than you originally borrowed?

On my personal loan, I never paid more than I should have in all three years I had it.... I took advantage of the Ramadam specials where they give you 1 - 2 months off on your payments to be added to your months at the end of your intallments... ie if you were supposed to finish paying on 1 June 2011, you will now finish 1 July 2011 or 1 August 2011, depending on how many payments you skip and how many years to took the option... They take a QR 50 fee for each month that you defer.... but my interest hasn't been affected... and even if it had, I doubt by thousands....