Here is the parts breakdown.
Replacement parts for Whirlpool ET18NMXAW02 REFRIGERATOR | AppliancePartsPros.com

The drain line comes out the back of the unit (Item 39 in Section 1).

Looks like it drains out the drain pan (Item 16 in Section 8) under the evaporator coils, then into the drain cup (Item 35 in Section 2) and then out the back of the unit.

If plugged a turkey baster with warm water, an air compressor with low pressure or a wire can often be used to clear it.

Also check that it is not plugged with dust etc. at the drain pan end.
You might as well clean the condenser/compressor fan and the condenser coils while you are down there.

If it looks like stuff is growing in the tube I would flush it with a 10% bleach solution. This should kill the stuff and let you flush it out. You may have to do this several times, let it sit for 15 minutes or so between flushes.

Be careful that the drip tray does not overflow and empty the drip tral after you are done.

Be sure to unplug the unit while doing this.