Here are your parts
Replacement parts for Maytag MSD2758DRW REF - SXS/I&W | AppliancePartsPros.com

Here is a service manual
http://appliancejunk.com/forums/index.php?action=downloads;sa=view;down=40
You may have to join the site to download the manual but joining is free and it is a great site for appliance info.

The usual way to trouble shoot is to force a defrost cycle and see if the heater comes on but in this case forcing a defrost (see the manual) is a bit of a pain. So I would check the heater/defrost thermostat combo (Item 12 in Section 4). It should be about 20 ohms.
A couple things to know is:
1. The thermostat opens just above freezing so check it when the coils are frozen up. Or hold an ice cube on it's face to freeze it.
2. Check to be sure the defrost thermostat is not deformed. If it is bulged replace it even if it measures OK.
3. Be sure to unplug the fridge when working on it.

If the heater/thermostat combo measures OK then I would replace the defrost control (Item 13 in Section 2)