Here are your parts.

GE likes to keep everything a secret so I cannot supply a wiring diagram.
The diagram for your unit may be in the machine compartment under the unit.

Here is an example of a side by side with a mechanical timer.
http://www.applianceaid.com/images/sampleSxSdiagram.JPG

Since you have 120 volts up at the heater/thermostat combo I would replace the heater assembly.
Looks like it includes both heaters and the defrost thermostat.

I believe it is wired defrost thermostat then heater 1 then heater 2.
It should measure around 20 ohms.
Note the defrost thermostat must be frozen to get a reading as it opens just above freezing.

I would not try running just a single heater, you will have a high current flow and it could weld the defrost timer contacts together.