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Parts for Frigidaire FRS6LR5EW3 Refrigerator - AppliancePartsPros.com

The drain tube (item 84 in Section 13) is connected to the trough (item 62 in section 13) at the bottom.
You have to remove the evaporator cove at the back of the freezer compartment to get at it.

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 tray after you are done.

Be sure to unplug the unit while doing this.