2002 Taurus blower problem

I got this vehicle 2 weeks ago. The heater only works with switch in "high" position. I thought it would be as simple as replacing the switch. After looking through some of the posts, it looks like it could be more complicated. Can someone please tell me how to determine what is faulty here? Thanks in advance.

A Haynes manual is helpful here. I recommend it strongly.

A single speed symptom is most probably a resistor assembly problem.

The unit is mounted on the evaporator case under the right side of the dash, near the blower motor.