I think that the challenge of refinancing a car loan, unlike refinancing a mortgage, is that the value of your car is likely less than the amount you have left to pay. Refinancing towards the beginning of the loan, when your car still decent resale value, maybe easier than refinancing now that you've had the car for a handful of years.
But here's an article that offers some suggestions on how to do it, including some lenders to check out:
http://www.chicagotribune.com/classified/automotive/used/chi-auto-refinancing-20120327,0,5760339.story
Is selling the car and going carless for a while an option at all? Not only will you save on the payment, but you'll save on all the associated costs of car ownership -- insurance, gas, maintenance etc.