Dunno about good, but here is some advice -
There are a variety of diaphragms and vacuum servos that are activated when you push the different buttons on the controls. These cause the flaps to move to the appropriate positions for heat, cool, floor, dash, etc. It is fairly common for one of these components to go south with age.
I would advise taking the car to shop that works on these cars a lot for a diagnosis before putting money into replacing a possibly good control unit.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe