REPLACE MY HEATER CORE

Your heater core is located behind your dash on the passenger side. It's not very easy to get to. To check if its operating, let the car cool completely. Open your hood. On the passenger side of the firewall, you should see two black rubber hoses going into the passenger compartment. Make sure that the vehicle is cooled completely, they can be scalding hot! Turn on the engine and put the heater on high, put one hand around each hose for a minute or two. One hose is the inlet and the other is the outlet of the heater core. They should both feel about the same temperature. If one is hot and the other is cold, you've got a restriction. Either the heater core is clogged or the heater control valve is not opening. Try having the coolant system flushed. If that doesn't correct the problem, you'll have to do a vacuum test on the heater valve diaphragm. If the valve doesn't open or the diaphragm doesn't hold vacuum, replace it. If the valve is working fine, you probably have a clog that can't be flushed. You'll have to pull out the core and replace it which is quite a hefty task.