Auto Air Conditioning & Heater Repair: heat on works on low, volt source, dodge caravan

Question
hello  i own a  1996 dodge caravan  and  i have replaced pretty much every thing  that  has to do with my  heat  even the  heat controls and  it  still only works on low but i had to  connect it  directly to  the power source   but i have  power  from  the relay on  fire wall  going into heater motor  and yet the  motor  works fine but  when i  connect  the controls  back up  it stops  working    my  thermostat is fine and  heater coil  is to  what  could be stopping  it  from  working  from  the  controls  to  the  motor

Answer
Penny,
I am looking at your wiring schematic as I write. But I want to clarify the question before I attempt an answer.
Your blower motor works only on low when everything is connected as designed, correct? You connect the motor hot wire directly to a 12 volt source and it turns the motor at high speed as it is supposed to? (bypassing all of the controls)

If I am understanding the question right....
between the control switch and the fan motor itself there is a resistor block. It is mounted on the passenger side of the engine compartment. This resistor block is what actually controls the different speeds. If you want low speed, the switch routes the voltage through all of the resistors prior to hitting the motor. Decreasing the amount of voltage, so the fan turns slower. On high speed, the full 12V goes to the motor so it turns fast.
Now from your description, it is acting in the exact opposite of the typical resistor block fault. Generally, if this goes bad, the fan will only work on high. BUT, if for some reason there is a short in the block or wires leading to the block, then I can see it routing all of the voltage through through the low speed circuit and acting as described.
So, in summary, check this resistor block and the wires leading to it. I am about 70% certain you will find this to be the culprit. If all checks out, then let me know and I will dig deeper. What you are looking for is any discolored wiring from heat or rubbing. The resistor block will need to be checked with a multimeter. Set it to measure resistance. You will then measure the output terminal to each of the input terminals. They should all be different from any other. If they are all equal, then the resistor block is bad for certain.

Thank you for the question, I hope I was of a little assistance. If so, please remember to rate this answer. If I can do anything further, either now or in the future, please do not hesitate to ask. Thanks again and good luck in everything