Is it possible for a hitch installation package to be the reason my heater no longer working? Wiring harness mess up?

Anything is possible -- depends on the parts used, the people installing it, and what might be tampered with. If it's a Jeep factory part and a Jeep factory tech -- I'd say a problem is unlikely. But, many "aftermarket" folks can tap into the incorrect circuit using a make-shift way to do things... requires some investigation and inspections unfortunately.