This resistor is essentially a variable resistor that controls the voltage going to the blower motor, thus regulating the fan speed. As the resistor deteriorates due to heat and age, it can start to lose its ability to regulate the lower speeds first, and eventually higher speeds as the damage worsens. The higher speeds usually draw more current, and the resistor might be able to handle it longer before complete failure.
Other less likely possibilities include:
* Blower motor itself: While less likely to fail progressively like this, a failing blower motor could eventually stop working entirely or at certain speeds.
* Wiring harness issues: A damaged or corroded wire in the harness leading to the blower motor or resistor could cause intermittent problems, but the progressive nature of your issue points more towards the resistor.
* Fuse (less likely): While a blown fuse could stop the fan, it would usually affect all speeds, not progressively.
* Switch issue (least likely): A problem with the fan speed switch itself is less common, but it could also be contributing.
In short: Start by checking and replacing the blower motor resistor. It's a relatively inexpensive and easily accessible part (usually located behind the glove compartment or under the dash). If replacing the resistor doesn't solve the problem, then you should investigate the blower motor and wiring.
