If it works on DEFROST mode, as Chuck Taylor said, that does bypass the other functions. That also means that you most likely have a vaccum problem somewhere in the system. If the system works on the "DEF" setting only, I would start by checking the vacuum supply line on the driver's side at the firewall under the hood, also check the lines at the servo (passenger side, lots of vac connections attached), also, if you feel daring you can pull the ACC panel out and check that the big vacuum plug is secured to the back of the pushbutton unit. Let us know and good luck!

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Regards,
Aaron