Rear heater blows only cold air no matter what the settings are at. Front heat works well. I have tried using both the front and rear controls to set the temperature, but the rear vents will only blow warm air at best. The air starts out as cold as the outside air even when the engine is warm and the front is blowing hot air. It takes several minutes for the air to get slightly warm.

SOURCE: Fully warmed up; blowing hot and cold; Suzuki XL-7

Its just a vent damper plate not operating, it's like a gate which controls the flow of air,,usually they are operated by vacuum, or cables.
Service center time, unless you are handy with your own repairs.

SOURCE: 2001 Suzuki XL-7. Heater blows hot one side, cold the other.

No ductwork [to the lower vents] under the dash. They are part of the heater core / blower housing is integrated (all one piece).

After reviewing shop manual, we suspect that one of the three damper flaps is not positioning correctly; or debris has blocked one side (which makes more sense given the symptoms).

We're waiting for the temperatures to get back in the plus numbers to check out the physical arrangement.