73 duster fan switch won't shut off

The fan switch in my '73 stays on and the switch feels like it might be junk. I have a '75 donor car and the switch has three solid positions (low, medium and high, I'm guessing) but mine just slides up and down with no clicking stops at all. I notice they both have a wired plug on the back. Are the switchs from the 73 and 76 interchangeable?