This is usually related to a failed vacuum check valve under the hood. It is connected to a large hose that runs directly to the throttle body. If it's stuck shut, the default is to direct the air to the floor plenums only, since you're not getting a vacuum feed into the control unit. Check by swapping in a known good unit or for a very quick check, remove the valve and install the hose directly onto the other line running into the car. If your modes work, that's the issue. If they still don't work, then the hose leading into the control unit is broken. If the hose is ok, then you're going to need to check the control unit itself (go to any pick-a-part, these can be had for a few dollars and I've rarely seen a bad one) and test by swapping it out. If that's ok, then it's time to check the vacuum motors and their connections and that can be a pig since you'll need to remove some interior components to get at them.