Car is moving the heater works fine but when I stop the heat turns cold?

Hello,

It sounds like you might have an issue with the heater core.
it could be partially clogged or restricted.
As you drive the higher engine speed pushes the coolant thru the heater core but when you are at idle the coolant flow is not strong enough to go thru the core, possibly due to restrictions.
You might be able to flush the heater core and that could improve the heat performance.

https://www.2carpros.com/articles/how-a-car-heater-works
https://www.2carpros.com/articles/replace-heater-core
https://www.2carpros.com/articles/car-heater-not-working

https://www.2carpros.com/articles/coolant-flush-and-refill-all-cars

I have attached diagrams for your reference.

Please let me know of any questions.

Thank you.