Ice behind back panel

I noticed ice in two of the top back panels. I unplug unit for 12 hours and then took off back panel to find a small amount of ice in top near fan. I have had no problems with cooling temps, freezer or standing water in drawers, I just noticed the ice. The drain heater was misaligned with a large amount of rust. A video said to use a copper #12 wire to keep drain plug clear. However, I am not sure if this was an issue as there was no ice showing in that location. So I am I am not sure how to proceed. Should I test heater and sensor? I am afraid to remove defrost heater since I have not had a cooling issue but I also do not want ice build up again.

Thoughts and thanks