Even simpler than the defrost heater, in an awful lot of fridges there's a little mechanical timer circuit that turns the heater on and off periodically. Those

Even simpler than the defrost heater, in an awful lot of fridges there's a little mechanical timer circuit that turns the heater on and off periodically. Those timers are full of plastic gears and can die, but while you wait for a new one there's likely an opening where you can use a flathead screwdriver to cycle around and manually trigger a defrost cycle. Just remember to set a timer and turn that back off after 20 minutes or so, because the same nonfunctional timer that starts it turns it off as well.