I have just received an e-mail from the car hire firm I use when photographing in Iceland offering a 25% discount on pre-bookings for the remainder of the winter and spring. The are called SADcars and are, reputedly, the least expensive in Iceland by quite a long way. The offer relates to booking made before 31st December 2014 for car hire in the period 1 January 2015 - 30 May 2015. To obtain the extra 25% discount just enter the code "2015Offer" in the coupon section of the online reservation form. Google for their website if you are interested.