I had the same problem. Every place I read said I had to replace the entire HVAC controlls. So I just dealt with having AC by feathering the switch someplace between 3 & 4. After 2 years of juggling this dilema I took everything apart and tried cleaning the switch. Long story short the switch is not the problem.
The problem was under the glove box.
It cost me $26 to fix the problem. Now I have all 4 speeds again and AC in all 4 speeds. No more flickering!!!! No more feathering the switch between 3&4 to get the AC's green light to come on. All works like it did when I got the car way back in 2003 with 3 miles on her.
I changed out the blower motor resistor.
www.rockauto.com
2003 MAZDA PROTEGE5 2.0L 2000cc L4 FI DOHC : Heat & Air Conditioning : Blower Motor Resistor.
Part # RU332
Price: $25.79