you have a fault. The centre vents as stated provide only fresh or cooled air (assuming you have AC) whilst all other outlets are under thermostatic control and should provide the full range from fresh or cooled through to hot.
My 300TE had the same problem before I bought it - the remedy was replacement of the electronic control panel of which the 2 thermostatic sliders form part (an expense fortunately met by the supplying dealer in my case).
90 300TE 24v (M104 eng) 207K miles
87 300TE (M103 eng) 269K miles (and never had the head off!)
86 300TE (M103 eng) 230K+ miles (donor car)
95 Toyota Lucida diesel 4wd
86 200T 165K miles (sold)