The defrost heater us usually located under the evaporator behind the back wall of the freezer. Remove the shelves, remove the screws and air baffels if it is a top freezer, then look to the bottom of the evaporator, there it is.

Something else you might check before you install the new defrost heater is check the continuity on the old heater before removal and check the defrost thermostat continuity. I have found many times the defrost thermostat is the real culprit. Check it by allowing it to freeze, that allows for continuity. Hold it in your hand and wait for a "Click" while it warms up to insure it breaks the continuity on its own. Thanks to Gene for adding the diagram I have not got the skill to do that just yet.