No power to A124 defrost recirculate and economy AC buttons

[Quiamare]

I have a 1994 A124 with 3 central AC buttons. I have a problem that there appears to be no power to any of the switches. It returned after I unplugged the switch unit from its loom and fitted it back but the following day no power again and I cannot repeat the previous "cure". I have checked the fuse 10 and all is OK. THe ignition is on motor running fuse OK but the switches do not work. Can anyone tell me where the power to the switches comes from after the fuse or what might be causing tis. The ac just blows hot without these switches working. Thanks