Defroster is working fine/ heater puts out NO warm/hot air when heater is turned on. Have tried to set on low setting with warm heat requested, then high fan setting with hot air requested, but nothing happens. Is a fuse or sensor out or is there a control valve setting or cable loose below the dash that opens a flap to allow this air to be forwarded by the heater? Need some help as this is getting to be a real issue. Thanks --Randy B