A quick test to determine if your push button control is going out on you is to remove the wood trim surrouding it: 2 phillips screws at base of wood just above stereo.
Then remove the 6 phillips screws holding the unit in. When it's not working pull it slightly out and knock on the bottom of unit with your knuckles. If it starts to come on again after a few knocks or so, you have a loose solder in there somewhere that might be repairable.
If not ACC unit, then look to blower motor, then regulator.
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi