There are two things I would check first...…..that hot water is flowing through the heater.....by feeling the inlet and outlet hose.....because there could be an airlock. As the engine warms, you will feel which is the iinlet hose.….
Also check the cable behind the dash, that the heater knob is moving the cable properly....you can easily drop the dashboard forwards....either three of four screws hold it in place.
Drop the steering column to its lowest point before you do this.
If there was no flow through the heater or the cable was not moving, why would it get hot standing in traffic?
All worth checking though.
