I have a 2004 Hundai Elantra and it has 70,000 miles on it.
The heat only seems to work if my foot is on the gas. I don’t have to be moving. If I am parked and I don’t have my foot on the gas then just cold air comes in. If I put my foot on the gas when I am parked and rev the engine a tiny tiny bit the heat begins to work.
Have you checked the coolant level?
Another possibility is that there’s a vacuum leak. Most cars operate the “doors” (gates) within the heating and ventilation ductwork by means of engine vacuum, and you’re not getting sufficient vacuum at idle. Take a look-see at the vacuum lines in the engine compartment for damage or leaks. I think one end will be at the intake manifold and the other end disappearing into the firewall somewhere.