If you are talking about the upper tailgate, one or both gas struts have lost pressure. They can the regassed but the cost of replacements is reasonable and a better option. To remove a strut, use a small flat blade screwdriver to prise up the lock tabs than just pull the strut off the pin. Also prise up the lock tabe when fitting the new one
When is the last time you change the thermostat and coolant?
Recommended interval is 2 years.
Coolant is the heat carrier to heater core.
Top off the coolant level for now and re-test.