hey I have a 91 ranger and when my blower motor is off heat from my heater core will seep in . but as soon as i turn on the blower only cold outside air comes in ,,,I took the heater core off and flushed it out ,,,Iv flushed out my engine ,,,,and iv replaced my thermostat,,,and still no change ,,I know that my blender door is operating fine because i can feel a slight difference when i switch between hot and cold, My A/C dont work but im not messing with that... But iv also operated the blender door by hand so i know that it is working right .....I know my truck runs cold but the heater should work better than that ....Oh yea and I do not have any leaks in the system ....does anyone have any ideas ???? thanks

wes