Rear console vents are for fresh air/AC only. The heat comes out of vents underneath each front seat. If cold air is blowing out of the console vents when the system should be heating then yes, you have a problem. I believe it is a simple cable adjustment. I had this problem in my old 300SD.
1976 300D (W115) - 330K and still going (sort of)
1991 300D 2.5 Turbo - Sold at 221K
1983 280SEL - Sold at 206K
1981 300SD - Sold at 232K