1990 chev 1/2 ton heater controlls

[roonie]

my 90 chev 1/2 ton heater controlls worked fine yesterday.since it got cold they do not.the heat button will not switch the floor to defog.the vent button works fine.also the off button has stopped working and the a/c button no longer turns the a/c on.these controls all seemed to stop working at the same time,since the weather got cold.the previous owner also had trouble with this same problem from time to time.thanks ahead of time.