You have a plugged defrost drain turn the airflow down to control freezing up in frig. To correct plugged drain you need to thaw it out and clear restricted drain

Found the drain, it was under 3/4" of ice. Drain was plugged and now flowing. Thanks for your help.

When you say "turn the airflow down to control freezing up in frig" is that the temperature control?

You give the age as 5-10 years, it ran OK for 5 years so you should not have to add a drain heater now.

Check under the unit at the drip tray to make sure dust etc. is not slowing the drain. Have someone pour water slowly down the drain while you watch it drain into the tray to see if it drains quickly.

Pour a 10 to 20% water/bleach solution down the drain.
Pour a little down give it 10 minutes then pour some more, do this a few times. Then take a turkey baster and force water down the drain. The idea is that you have crud growing in the drain tube, you want to kill it and then flush it out. I would clean the drip tray before doing this and then you will be able to see if you did flush out some crud.

The above only applies if this unit drains directly from the freezer compartment to the drip tray. Some units drain into a funnel in the back of the fridge section and then down to the drip tray.

Have seen where some people take some bare 14 or 16 gauge copper wire, wrap it around the defrost element a couple times and then have it go to the drain. The idea is the wire will heat up during defrost and clear out any ice at the drain.
I am not a big fan of this idea as it will add a cold spot along the heating element and may shorten the life of the element. This is just a gut feeling as I have no experience doing this perhaps someone else will jump in who has tried this.