It started with no heat getting into the car. I was told to check my coolent, which I did. It did not work so the next thing I tried was changing the thermostat. It also did not work. Finally I looked up you tube and I saw a video on how to flush the heater core. It suggested that I disconnect the inlet and outlet pipes that supply water to the core and to flush the core with a hose pipe. I did and a lot of dirty water came out. Some rubber also came out. Now I see the water is leaking into the passenger side of the vehicle. Do I need to replace the heater core? And if so, how do I do it.
First do a pressure test on the system. Next, if the car has air conditioning, there is a drain pan and tube that is likely blocked with debris. The condensate from the AC system collects in that pan and is supposed to drain onto the ground. Coolant from a leaking heater core should also drip into that pan and onto the ground, not on the floor of the car. Look for a 4" long rubber hose with a 90 degree bend hanging down on the passenger side of the firewall under the hood. Squeeze that tube to open it up and pass any debris. If nothing comes out, squeeze the wire ring that holds it on, pull the tube off, then carefully stick a pencil into the spout to clear any blockage.
Your car really isn't old enough to have a corroded heater core yet. If it is leaking, suspect the coolant wasn't replaced every two years. It is replaced because acids form in it over time, and the corrosion inhibitors and water pump lubricant wear out in about two years.
Replacing the heater core is a pretty big ordeal. It's not really a do-it-yourselfer project but if you want to attempt it, get a copy of the manufacturer's service manual. That will have pages of instructions with drawings. You'll need a mechanic to discharge the air conditioning system and recover the refrigerant. When you're done you will need him to evacuate the system, then refill it and check it for leaks. Refrigerant is very dangerous to work with. At a minimum, AC specialists wear gloves and safety glasses, and most wear a face shield.