2002 4Runner takes 10 minutes for heat to turn on inside the cab

I'm hoping you can help me. First off thanks for this forum.

2 issues

I have a 2002 4Runner SR5 and recently the interior heat, even though the fan is blowing, doesn't turn on for a good 10 to 15 minutes. The temp gauge on the truck shows normal but it still blows hot air until all of a sudden the heat kicks in.

Additionally, the rear heater, that has its own control on the console for people in the back seat, turns on but won't ever get hot, just blows cold air.

Any thoughts on what I can check or fix would be most helpful. It was 6* this morning.

 

When was the last time you thoroughly flushed the coolant?

Drain it, put in some kind of commercial flush chemical, run it according to the instructions on the flushing stuff, drain, fill, run, drain, and fill it a few more times with regular water, to get the flush out, then fill it with the Toyota Red coolant and distilled water, 50/50 mix. You can use water from the hose to do the flushing, but make sure it's drained well and replaced with distilled for the final fill
Make sure you burp it well on the final fill!

Might be a good time to replace the radiator hoses, and maybe the thermostat. At least test the thermostat on the stove.
Inspect the metal pipes and rubber hoses running to the rear heater under the truck. They are known failure items.