I have a 1979 Mercedes 300 SD Turbo Diesel. The heater will not work. I've replaced the Thermostat, checked the blower motor and made sure the water was getting through the thermostat. My rapair manual has nothing in it about the heater or heater core. How can I locate and check the heater core and/or what else could cause the heater to not work? Also, is there a heater shut off valve like larger diesel vehicles have? Please help.