Car: 1991 560 SEL 104100 miles
Problem: Can't get heat from heater
Setting: temperature wheel with 75 F in the middle, outside T reading 32 F, coolant T reading 82C, fan set at auto, control set middle one.

Observations. Fan will run at max all the time, however, very cold air will coming for about 3 minutes to the foot area. After that, fan keeps running at max but nothing seems to come out of anywhere. Same if I use the setting next to the defrost.

Please give me some tips as what and where to check first. Many thanks in advance.