Start with the basics:
Fuses F10 (control) and F11 (power)
Then go to the most common component: The pushbutton controller
On mine, and most reported here, the internal circuit boards had cracked solder joints. This is due to the boards arranged in an "H" and over time the solder cracks where they meet. A search of "resolder pushbutton controller" will likely give you pictures of the areas I'm talking about. If you aren't comfortable with resoldering, there are others here who can do it or remanufactured units are available.
Usually a vacuum pod problem is when the system doesn't direct the heating/cooling to a particular vent but not all of them. Defrost and heat are the defaults.
Bottom line, the "defrost only" is likely to be the Pushbutton Controller and "no heat" is likely to be the mono-valve.
Here are some diagrams to help you out:
https://www.startekinfo.com/StarTek/outside/12253/program/ETM/81etm.pdf
https://www.startekinfo.com/StarTek/outside/12253/program/Climate/83-604.pdf
https://www.startekinfo.com/StarTek/outside/12253/program/matrix.htm