KJ,
Try this link for a lot of great info on car rental savings:
http://www.mousesavers.com/groundtrans.html
We are also going at the end of next week and ran into problems using the codes because President's week is a blackout period for most, if not all, the rental agencies. Not sure when you are going but just wanted you to be aware that most codes will not work if you are picking up between between now and Saturday, 2/23. They will work from 2/24 on, so we are renting a full-size from the National Emerald Aisle, using the Disney Club discount and Disney Connection coupon for a free day, from 2/21 to 2/24, then returning the car to the airport and picking up an SUV from Dollar for $209 for the week from 2/24 to 3/3 (only available using a code from the website:
http://www.floridaticketsinc.com/dollar/index.htm).
You may not want to go thru the hassle of returning your car to the airport and exchanging it, but if you are staying for a week or longer it may be worth the hour or so it will take to drive to the airport and back and make the exchange (though most of the agencies also have locations in or near the WDW area where you can make the exchange if you use the same company for both rentals - we did this last year with Dollar and it worked well). The $ savings was definitely worth it for us!
Hope this helps.
Ted