The horn on my 2005 kia rio works sometimes and sometimes not. What is the problem ?

Could be that the clockspring is going bad. The clockspring in the steering column allows for constant electrical contact no matter what the wheel position. When clocksprings go bad horns stop working or work unusually, etc. This is a common issue with the newer cars, after a while dirt/dust and rust makes the ring make contact with the triggers or they dont pick up the triggers (Horn wont work)

If it has been happening since you got the car, have you taken it back for them to look at? If you have there will be records of that and see if you can get a "good-will" repair on it. If you haven't mentioned it to the dealer, they will repair it but you will have to pay. There arguement will be "If it has been doing ot since you got it why has it taken you 3 years to bring it in?" But as seeing it is a 2005 there still might be some Factory warrany on it, up here in Canada 160 000kms (100 000 miles) is the standard warranty. I do beleive that KIA in the US has a 10 year 100 000 mile warranty. You might want to check in to that. Best of luck, Hope this helps.