A model number may help others help you.
Find your model number - AppliancePartsPros.com

I have never tried weed eater string but it should work. Problem is that you cannot flush out the plug. Also you cannot tell if it is card until the unit does another defrost.

Sounds like drain tube is plugged so that the water from the defrost cycle cannot drain into the drain pan.

I would check that it is clear.
If I read your post correctly it sounds like this is a side by side unit so odds are high that the drain is under the evaporator coil in the freezer.
You will have to remove the evaporator cover5 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.