If you pull off the lower bezel and upper bezel around the radio, you'll be left with the HVAC controls, which are held in by 4 screws. You can take the screws out, and pull the unit out. The settings where it blows is done by way of vacuum lines. Most issues with that are due to if it's taken out and disconnected, and not seated right putting it back on, can give problems. There's one main feed line to it, black I think, which goes to it through the firewall. I've seen on vehicles where that can be an issue if there's a vacuum leak happening. May not be the issue, but good place to start. Then work your way from there.
I'm thinking it sounds like as the motor is creating more of a vacuum at higher RPM's, it's causing to leak somehow, maybe a small hole or crack, which then causes a vacuum loss and then everything goes through the defrost, which is the default, as without vacuum, whatever setting you want will now not work.
The beatings will continue until attitudes improve!
HID Quad bi-xenon + foglight retrofit