'92 Eldorado Driver's Side Heat Issue

Hi, I'm going to be working on a '92 Eldorado tomorrow, the owner is complaining of very little or no heat coming out of the driver's side vents, but good heat on the passenger side. I have worked on the car before and know that it doesn't have dual climate control. The only think I can think of is that the antifreeze is low and the heater core isn't full, which is allowing the warm air out on the passenger, and by the time it gets to the driver's side, it's cold? Make sense? Does anyone know if they have a separate door for driver vs passenger side air ducts?