Sub Zero Refrig icing up

Have a Sub Zero Refrig / Freezer - Dual compressors at very top, refrig in center and freezer drawer at bottom. Likely about 15 years old +/-

Left back section of refrigerator gets iced up - evidentially defrost coil not heating in that section. Suspect its either timer or bad coil.

Does any body have photographs of one of these units opened up? I don’t want to waste time trying to remove panels I don’t know how to access. What would you check first and how/

Thank You

 

Do you have a model number?