1990 Jaguar XJ6 Air circulation

My vehicle has the automatic climate control option Both blowers on the car are working, as it blows air at all 3 fan speed settings. When I am running on the economy setting (air conditioner off), when I set the temperature dial to a cooler setting, the system shifts the air flow to the center vents on the dash, and when I move the dial to a warmer setting, the flow shifts to the side vents on the dash, and heated air flows through these vents. This is how I believe this is supposed to operate. My problem is that in the economy setting, when I move the dial to a warmer setting for heat, the air going to the floor vents in the footwell and back seat through the console does not warm up and seems to be whatever the outside air temperature is. This means, of course, that I get cold air through the floor and rear vents in cold temperatures. When I am running the air conditioning in warmer weather, there is cold air through all of the vents. What is causing this problem?