Here's the parts diagram for your model:
(click to enlarge)
#21 is your defrost drain pan, which is located in the back of the refrigerator, once you pull the refrigerator out from the wall, you will see if down by the compressor assembly #8.
See if any water is in it, or if its bone dry.
If its bone dry, then remove the back panel inside the freezer and you will see your defrost drain trough #7, below the evaporator coil #20, and you will see the defrost drain hole in it. See if that defrost drain hole is clogged up, run hot water down it and then run something down the drain hole, then run a little bleach down it.
I use a evaporator cooler spider snake to run down that defrost drain hole: http://www.azpartsmaster.com/Products/Cooler-Spider-Snake__CLR4690.aspx
They are used basically in Arizona to clean out the water pump tubes that run to the cooler pads.
I don't know if they would sell them in your area, but if you go to your local hardware store, they should have something similar to run down your drain to clean it completely out.
This video will give you the general idea of how to remove the back panel inside the freezer:
Repairing a Whirlpool refrigerator? This video demonstrates the proper and safe way to disassemble a refrigerator and how to access parts that may need to . Learn more from the experts at Repair Clinic.
Let us know what you find.