Depending on what kind of car you have it is most likely the heater core. It looks like a small version of the radiator. As well there is probably a fan under there if it is not located on the firewall. On older vehicles (60's/70's) both these items are sometimes accessed from the engine compartment
how to take out the heat on the dashboard
It should if the system is in good working order.
Under the passenger side of the dashboard.
Yes they have a heater/defroster.
Most likely you have a vacume leak under the dash.Check for lines off or broken.They will harden and crack with excessive heat,such as in the engine compartment.
The ac unit compressor will run when it is in defroster mode and heat setting is on.
do you have a heatpump or a/c? if it is a heat pump you can,that is what happens when the system goes in to the defrost mode. if its freezing you could be low on charge or or bad defroster board or even worst a bad reverseing valve.
CHECK BLOWER MOTOR, AND HEATER CORE BOTH INSIDE THE CAR UNDER THE DASHBOARD
Dont know what the problem is but i can tell you its a vacuum leak . the vents are vacuum operated and require air pressure to change between the vents and floor. the defroster is default setting you may have a vacuum line that has come undone but all i know is its somewhere under the dash.
What type of auto is this? I may be able to help. Let me know.
Radiation heats up a dashboard kind of a like it does the earth. when light from the sun passes through the window and hits the dashboard, the dashboard absorbs the heat energy, and re-radiates it into the air. this new energy is not as strong as the radiation from the sun, and doesn't pass through the window as well, and is trapped in the car. the trapped energy is what causes the dashboard to heat up.
Not unless your heater core leaks.