If you roll under the vehicle you will see 4 aluminum lines going to the rear down along the passenger side of the vehicle. Is the coolant/lines in two of these lines hot? Have you checked the coolant level in the radiator when the coolant is cold by removing the cap and checking? Or are you strictly going by the coolant level in the reservoir bottle?
If you're only topping off the reservoir you need to get to the actual radiator cap that holds the pressure in the system and make sure that the coolant is full in the engine and radiator. This vehicle is known to have major leaks in the radiator. If the heater is not blowing hot in the rear it usually is because of coolant deficiency because this is the furthest for the coolant to travel. Putting coolant only in the plastic reservoir that is incorporated into the cooling fan housing is not sufficient.
The radiator is known to leak down along both sides. It was actually recalled in 2004 to replace the radiator. Even the replacement radiators for the recall I have seen to leak a couple or three years after installation. There are no shutoff valves for the coolant flowing to the rear radiator/heater. The only thing that controls hot or cold air is a blend air servomotor that bypasses air around the heater core.
If you have ruled out that the coolant is circulating to the rear the only other possible option is that the air blend servomotor or controls inside the heater box in the right rear quarter panel area is malfunctioning. To gain access to this heater box assembly is quite a project. Have you double checked all the control settings front and rear? Does the heater control low straightly colder if you switch it to the air-conditioning mode? If you try and switch both heater controls from cold to hot rapidly several times can you kick it in to make it work for heat?