94 e 150 Heater Control Valve
 

Cold air coming from front heater/defroster but rear heater works great. Suspect heater control valve-where is it?

Also front fan doesn't produce a lot of airflow. Both fan switches seem intermittent. I used to have 85 Ford van that needed resistor replaced. What is location of resistor(s) for testing?

Fan Speed Control
 

I would look on the blower plenum. You should see an electrical connection plug. There should be a couple of screws holding the resistor speed control. Disconnect the battery negative terminal. Unplug the harness, remove the screws. If any of the coils are burned it will keep the blower from operating at that speed.
To produce full speed, the motor must have good connection at all location through the switch and to the battery and fuse panel.
Corrosion will cause heating at the joints. Heat keep the proper current from turning the blower at full speed.
Good Luck
baribob