Sounds like the air "blend" door might be inoperable. This door opens and closes a specified amount to regulate the amount of either cooled or heated air, as requested by the climate control unit. If the door is continually in the position which is bypassing the heater core (cold setting), the air will not get warm regardless of the condition/operation of the heater core. I'd suggest having this component checked.
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sounds like your blower motor is OK so either the hot water is not getting through the heater core, or the air is not being routed across the heater core. If the hoses or core are blocked, that would do it or if an air door is not allowing the air to the core.
Hopefully it won't come to core removal, but this video should illustrate what is going on. There are also videos for unplugging the core. good luck
There should be a actuator valve on the line that determines if the heater core gets hot water or cold water. When the heater is selected, the actuator opens and lets hot water into the heater core. It sounds like that actuator is corroded and not opening.
1st of all, you need to check your anti-freeze level. If its not full, the coolant cannot/will not flow throughout your heater core which the coolant flow through to heat your vehicle. If it is indeed up to the proper level? Than, yes I would look into the thremostat. It may not be opening up.
Most likely the problem is a broken blend door inside the heater box or a broken adjuster cable going to the blend door, the blend door inside the heater box is responsible for heater output depending on your temp adjustment on your heater control panel, most likely the blend door is stuck in the cold position allowing cold air to come through the vents verses allowing the blower motor to blow warm air past the hot heater core to produce heat, check the temperature adjuster cable behind your heater controls and make sure its connected and not broken, lastly make sure the blend door isnt broken and operating freely.
Have you checked the coolant level? What you describe is exactly what happens if the coolant level is low. When stopped at idle, hot coolant is not circulating through the heater core, so no heat. As the engine increases in rpm's when you pull away from the light, the water pump is spinning quicker, and some coolant is making it to the heater core. So check the level, and if it is low, keep an eye on it to see if it drops again. That of course indicates a leak somewhere. Get it fixed, as overheating the engine leads to costly repairs and worse case a new engine. Let me know how you make out with this.
The rear vent system has a separate heater core/blower system. heater core could be plugged up or the coolant lines going to rear heater core are plugged. could also be low coolant/faulty water pump not producing enough pressure, or air in the cooling system which will need to be bled if so.
if you are sure that your heater core is not restricted. then try to recalibrate the climate control panel. if your a/c system is not working then DO NOT ATTEMPT THE RECALIBRATION it will make your lights on climate control panel flash codes forever.
to reclaibrate the climate control panel just have the engine running. turn the mode to vent, the temp to cold, the blower on high, then press and hold the rear wiper wash and wiper buttons at the same time until the panel lights up then wait about 2 minutes for it to run completly when completed. turn off ignition and then restart the engine try to se if you have heat again. if it doesntwork post the codes it gives you after the calibration test.