2001 Windstar do not blow AC/Hot air from rear vents.

My 2001 Ford Windstar SEL do not blow either cold or hot air from rear vents. Doesn't really matter whether I use Rear control mode or try to control from front controls. Looks like the rear fans are not working from the ceiling as well as from the floor. The air comes out fine from the front vents on the dash board without any problem. Can someone guide me in the right direction where to look for the problem?