Climate control switches intermitently works.

If I turn my AC on I have to push the DEF switch or else the AC will not turn on. THe DEF turns the blower on all the time, it works. When the DEF turns on, I push the Bi-level and it will turn on but it goes off and on. The Hi and Lo sometimes will come on but also goes on and off. So for the AC to work contantly I have to use the DEF. Switch.