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Replacement parts for JENN-AIR JSD2789GES REF - SXS/I&W | AppliancePartsPros.com

You probably do not need any parts.
It sounds like your drain tube (Items 5 and 7 in Section 1) is plugged.
I think it attaches to the evaporator housing (Item 14 in Section 4).
The water from defrost cannot drain out so it collects in the freezer.

Remove the evaporator cover inside the freezer.
And check that the drain is clear.
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 will probably have to pull the unit forward and remove the rear cover to do this.
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.