How do you repair a car air conditioner that works intermittently?

My car air conditioner sometimes works, sometimes doesn't. When it works it does blow cold air and the control nob works to control the temperature.When it doesn't work, no air comes out, but the the lights on the control nob do come on. I narrow down to replacing three things. 1. The blower motor 2. The... show more My car air conditioner sometimes works, sometimes doesn't. When it works it does blow cold air and the control nob works to control the temperature.When it doesn't work, no air comes out, but the the lights on the control nob do come on.

I narrow down to replacing three things.

1. The blower motor
2. The blower motor resistor
3. The blow motor relay

Which one of these things should I replace?
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