I just replaced the pushbutton unit on an 83' 500sl with a refurb from mercedesshop. The blower never came on before. After replacing the unit the blower does work. However when the blower is running, regardless of which button is pushed (except off) air comes out of the three center vents. Very little air comes from the footwells. If I close the center vents the air seems to be diverted to the defroster vents. There was an aftermarket stereo installed when I got the car. During installation could they have damaged something. Or does this sound like a more common problem. I've pretty much accepted the fact that I'm going to have to replace everything in this car. So what's next?