Defroster Stuck Open At All Times W/Heat

[Kels02c240]

Has anyone who owns an 2002 c240 had any problems with their defoster sticking open at all times with the heat on? And if so how did you correct this problem? when you switch the control for the floor and vents etc... it works fine but the defroster overrides the a/c with the heat.when turning the fan speed up it also comes out more out on the defroster as well and less out the the vents. Any input would be great Thanx