[kuehl]

The primary load is the motor, the switch contacts see a lot of amperage.
The secondary load are the 2 resistors in the fan speed resistor unit.
The switches do fail over time simply from the load, and how old is your car?

First, you can test the evaporator motor by running leads off the battery our using an alternative 12v power source. Let the motor run for 15 minutes, if it continues to run that is a simple test. Next, if you have access to a simple inline amp meter (old school analog, 0-20 amps), see how many amps it is drawing after 15 minutes.
Typical amp draw a full speed is between 12-15 amps, the load will be higher when the evap motor and blower wheel are in the box as opposed to out of the box.

Second, you can inspect the fan speed resistor unit's 2 contact point sets, they bend as heat from the resistor springs increases to turn things off. Here, Discseven had a thread on the topic, Karl. please post the link.