Two days ago the heater stopped heating the cab in my 92. There are no coolant leaks. I run 100% Prestone antifreeze. The engine warms up and runs at normal temp. Radiator is full. I've looked under the dash and all of the control knobs and linkages operate normally. I removed the in-flow hose at the heater core and blew through so I know it is not clogged. I checked the heater core nipples after driving and they are 160° each. The fan works on all speeds. Distribution works, face, feet, defroster, just no heat! HELP! What am I missing???