This is either of three defrost parts. The timer, heater or defrost limit switch. Most amana's I fix have a bad limit switch which is clipped to the freezer coils directly. Spin your defrost timer till it clicks and check the same frost laden wall to see if the frost starts to steam and melt. If not, unplug the fridge, then try disconnecting the limit switch and join those 2 wires together. Replug and see if the heater starts steaming. If it doesn't it is most likely the timer. The heater is usually the last thing to die, unless your heaters are encased in glass. In which case a quick visual inspection for decay and separation will pinpoint the problem.