rear vents only blow cold

99 tahoe 4 door 2 wd 118k

my rear vents blow cold with a/c and "cold outside air" when it's on heat, no matter how i set the front, were about to take a road trip and I'd like to fix it B4 we go. there is an indepenfdent fan control switch in the back, fan control works ok, i just wont blow warm.

any tips appreciated

Rear temp control door inop and/or rear heater core plugged.

the back dosent have its own temp control, its supposed to be the same temp as the front. it has its own fan control and the fan control works fine. if the back dosent have a seperate temp control would it still have its one heat core?