I haven’t seen every variation of these but quite a few. The ones I’ve seen use a small relay for high speed and series resistors for the lower speeds. If the relay got stuck ON, it could continuously run on high speed regardless of switch selection. The relay is located right on the resistor pack card on the type I have encountered. Pull the resistor pack out and use your meter, but measure resistance rather than voltage to see if the relay is bad. I actually repaired the one from my Trailblazer. It blew out a trace when squirrels decided to store nuts in the cage, That overloaded the motor and blew a trace on the resistor card. Had to remove some of the conformal coating to expose the traces and repair it but much cheaper than the $55 the part cost new. Been working fine for years ever since then.