It's best to diagnose the problem before making that decision. Diagnosis is easy, especially if you have a glovebox rather than a passenger airbag. If you remove either - glovebox is easier and less likely to explode - the switchover valve is staring right at you. From that point you can test each of the seven vacuum lines running to the various actuators in the system. A small hand-held vacuum pump (AKA mity-vac) is the way to go here.

On a 124 the defrost actuator is the only one you can replace without removing the dashboard. However, if that's your problem, I would suggest changing only that one. Again, it's perhaps a 30 minute job.

Given the age of your car, most likely you'll find two or three of the seven vacuum lines leak down. In that case, the dash will require removal to replace the failed vacuum actuators. If you're going to pull the dash, then just replace them all. The additional parts and labor cost is minimal relative to the expense of pulling apart the interior of the car.

My $.02,

- JimY