GE SBS Freezer Cold - Fridge Not

I was given a GE side by side that is about 20 years old. It worked for about 2 weeks then I noticed the freezer was about 0F but the fridge is around 43f to 47f… I took off the inside panel and the coil was frozen solid. Not light frost but had frozen over. I saw on the net it may be the Defrost Thermostat but after checking it, it tests out ok. Problem is the back does not come off to check anything else. The main thermostat seems to cut on and off the compressor. The fan at the top of the freezer is working. I’ve not been able to see any other fan. Any help would be greatly appreciated. I can’t afford another fridge so I need to try to fix this one. I have noticed due to age the parts are not really easy to find.

Thanks in Advance,
Jamie Dean

Temporarily short the defrost thermostat, turn the defrost timer till it goes in defrost. If it now defrosts, replace the defrost thermostat, if not, check the defrost heater.