Muddy water could be a source of a clogged heater core. Remove the hoses at the heater core and flush out with water pressure. Replace the thermostat with a 195 degree thermo because it may not be working anymore from the muddy water. When you have the upper hose off at the thermo remove the lower hose and water pressure again thru the radiator. Are you sure you have a properly functioning blend door actuator or any problems with dash controls? Like does the heat only work at defrost but not at the floor....etc.?