Need some help. Just replaced the heater core in my 99 a4 2.8. When to use the heat and only get heat out of the center vent. I switch to the defrost and I lose heat and still blows out the center vents. Can you have the plugs switched?

 

If I remember correctly, I think you can pull the carpet back on the passenger side (where the center console meets the footwell) and see the actuators. If that's true, you can run the heat and see if the actuators are working when you switch modes.

I think the easiest first step however, would be to run your A/C (of just the fan) and see if you're getting the exact same results. If you are, then you have a problem with either the control module, the ducting itself, blend door(s) or actuators.

G/L

 

Did it still have the foam covering on the metal flaps inside the box?
If not, you will have to take the box out again, take it to bits & goue a sheet of closed cell foam on each side of the flaps, & put it back together again.