Hello, I'm Danny.
There are two things I would check for the blower motor condition you are experiencing. You should check both the blower motor resistor and blower motor speed control that are both mounted to the under dash hvac box. Here is a tutorial showing how to test for electrical wiring:
I would check the wiring harnesses for corrosion, being melted, or burnt. I've attached picture steps below on how to replace both the blower motor resistor and speed control including their location on your truck. Hope this helps and thanks for using 2CarPros.
Images (Click to enlarge)
Tuesday, June 1st, 2021 AT 4:51 AM