What is the temperature in the freezer?
Should be between 0 and 5 degrees F.

Before starting make sure that the drain line is clean and not partially plugged which would slow down the draining.

If the temperature is in this range.
Check that the drain pan and area where defrost water would flow is nice and clean. Any dirt or crud build up can cause the water to drain slowly and then freeze ate the drain tube.

Sometimes a buffed up wax coating will help the water drain quicker.
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If all looks OK the you could try the following.
Get some 14 or 12 gauge solid copper wire. Wrap it around the defrost heater once or twice ( snug but not tight) and then route it to the drain hole. It should go about an inch down the hole.
Now when the evaporator defrosts it will also defrost the drain.

It is a good idea to force a defrost cycle just to be sure that the wire does not get too hot.