Help with heater. Front heat doesn't work rear does

My daughter's 2004 X5 currently has heat in the rear vents. However, it is only Luke warm when switched to defrost or front vents. It's winter time in Alaska and really need to get this fixed.
Would it be the heater core? I figured if that were bad the back heat wouldn't work.
I have disassembled under the dash to gain access to the vent tubing and fresh air door, it works and switches the vest when the floor to vent is selected.
I have also replaced the HEATER CONTROL VALVE but still don't have heat to the front

Can anyone please help with this? Thank you.

Only half of your heat works in your car.
It may be because of a broken or faulty thermostat. Sometimes faulty thermostat is the most common cause of your car's failing heat. You can fix this in your near workstation by consulting an experienced technician. Hope I am able to make you understand engine heating, if not plz view this-

https://www.usedengines.us/why-is-your-car-engine-overheating-when-the-coolant-is-full.php