Whats the problem when the ac blows from the defroster instead of the vents in a 1993 Lincoln mark 8?

I had the same problem on my 95. Check the vacuum hose that goes from the center of the intake manifold toward the passenger side of the car. (You have to remove the intake air box that heats incoming air which is on top of the intake manifold. Pretty easy, just a few nuts and a clamp) Mine had gotten old and was cracked. The default mode for lack of vacuum on the blend doors is the defrost setting. I picked up a length of vacuum hose and connected it up. Problem fixed.