99 Cabrio a/c interior circulating fan problem

My 99 cabrio interior circulating fan does not work in the first three speeds. It does work on the highest speed. Now for the confounding problme....when I turn on the a/c. Fan still does not work in the first three speeds, but when I turn the fan to the highest speed, the a/c turns off!
I'm thinking the fan switch is bad, so could replace the a/c control unit, but don't know if this will fix the problem.