Ice buildup behind freezer wall

Our 12 year old Whirlpool fridge developed this problem Friday, and a partial diagnosis is our defrost heater is bad (no continuity) causing ice to build up on the coils behind the freezer's back wall and block airflow. I can obtain this part myself for around 100 bucks at a brick and mortar, or wait a couple days for online delivery for 60 bucks, but if this is not the ONLY cause of this problem, I would rather just get a new fridge.