Either a leaking hose going to the heater coil or a bad heater coil. Water gets inside car on front floorboard.How do I replace heater coil?

To be honest, most mechanics hate replacing heater cores because of their location. They are usually on the firewall on the inside of the car underneath the glove box under the dashboard and behind a plastic black cover. If you look under the hood at the firewall on the passenger side, you should see on inlet hose and one outlet hose which should be replaced along with the heater core. They must be removed first before you can remove the heater core. Basically you have to lay on your back on the passenger side Front) floorboard, remove the necessary panels to get to the cover, remove the heater core and install the new one first, then intall the new hoses, reinstall the cover and any panels you removed. Lastly, be sure to bleed and top off the coolant while you run the heater, then check for leaks