Here are your parts, I think.
I could not find info on a 15556 the following is fora 1556.
Replacement parts for MAYTAG MQU1556AEW FREEZER- VERTICAL | AppliancePartsPros.com

Check tor 120 volts at the fan when it should be running.
If it is there and the fan is not running the fan is toast.

Check the drain tube at the drip tray to ensure that the tube is not partially clogged up with dust.
Use a turkey baster etc. to flush the drain tube with a 10% bleach solution.
Do this a few times letting it sit for 15 minutes between flushes.
This should kill and hopefully flush out any crud growing in the drain tube.
Just be careful to not overflow the drip tray.

Try waxing the water path to the drain. A couple coats will do. Buff it up and see if it fixes your drain problem. The wax will help keep frost from forming here and slowing down the defrost water, also the water will drain faster.

I do not know exactly where the drain hole is so the following may not be feasible.
If the drain is behind the evaporator cover take some bare 12 or 14 gauge wire and wrap it a two times around the defrost heater and then down the defrost drain a couple inches.
You now have a drain heater.
This has worked for many freezing drain problems.