HiCustomer Thank you for asking your question on Just Answer.
Can you tell me the Model of truck that you have?
If you are not sure, you can send the Vehicle Identification Number (VIN), and that will tell me.
It is a good chance that the problem is the blower motor resistor. Here is the schematic for that system.
But before you try to change the resistor pack, you might want to check the High Speed Relay. If it has a bad connection on one of the contacts, it would have the same symptoms at the resistor pack being bad. Here is a diagram of the relay connector.
If you remove the relay and connect a jumper wire from connectors C1 & B2, then the blower motor should work on the lower speeds IF the resistor pack is OK. Then all you would have to do is replace the High Speed Relay and that should fix it.
If the blower does NOT work on the lower speeds after putting in that jumper, then the resistor pack is probably bad.
Here is a diagram of the components of the system.
You MAY be able to get to these components by removing the insulator panel on the passenger side and the glove box.
I hope this helps. Please let me know if you have more questions.
Take Care, Greg A.