Sephia Door Locks
I fixed this problem on the driver's side of my 2001 Sephia last fall. I eventually took off the door panel to inspect things more closely but actually fixed the problem without removing the panel.
Roll the window all the way down and jam some spreaders between the inside of the window glass and the weatherstrip, at the latch end of the door--I used a 3" putty knife. Now there should be enough space to shine a flashlight down inside the door, and after adjusting the position of the light, your spreaders, your eyes, etc., you should be able to see most of the mechanism. All this works best when it is dark, either in the garage with the lights off or at night.
Now insert the key in the exterior lock and turn it back and forth a few times, so you can seen what the mechanism is doing. Since the lock won't turn to the "unlock" position you won't get full travel in the mechanism, but you can work it back and forth between the key and the inside lock lever. With some luck, you can lubricate the mechanism by threading a long thin tube into the space between the glass and weatherstrip, and squirting oil into the tube. I used 1/8" clear heatshrink tubing for this. Where to lubricate? Follow the rust. I don't think it is possible to over-lubricate it.
It appears to me that the root cause of the problem is that the mechanism is directly in the path of water that drips off the lower edge of the window glass--a really weak design. So if your car isn't garaged, this problem is going to happen on both driver and front passenger doors. Probably happens to all of them eventually unless you never get the car wet!
If you want to get into the door to do a more complete job of restoring operation--and perhaps make a shield of some sort to prevent more water from getting into things, you have to get the door open first. After that, the non-obvious screw to remove is behind a ~1/2" plastic button that is in the back of the interior door latch cavity. You also have to work the trim piece off of the latch area. The other problem is the window crank. A C clip, between the door panel and the crank at the base of the shaft, secures the crank. It isn't easy to see. It is best removed with a GM tool that is what kia recommends and is almost universal for this kind of clip. I got one for about $5 at NAPA, not knowing exactly how difficult it was to remove. After I was done, I decided I could have done it without the tool, but as usual the tool made it easy.
Now the passenger side door has developed the same problem, and I didn't get to it before another driver locked the door. It is really stuck. I've made a couple of attempts at lubrication, but no luck so far. I'm contemplating cutting a hole in the door panel if I don't get results soon.