1990 300SE, that would be the W126 chassis.
Two seperate problems:
First, poor heat. Reguardless of where the air is blowing, if it is lukewarm when the temp. wheel is on full heat the first place to check is the mono valve (heater valve). It is located near the battery just in front of the windshield. There is a little two pin plug on top of it. 4 screws hold the top on it and there is a rubber diaphragm that probably has a hole in it. Its a 5 minute job if you don't drop the little screws down the firewall. Very common.. always the first place to look.
If it was not torn, and you don't have a thermostat problem it gets more complicated.
Second, as mentioned by someone else, pull out your owner's manual and verify the function of the air vents. You will not get any warm air out of the center vent in these older body styles. But in a heating mode, you should get air blowing from the side vents and the footwells flaps. If not, you may have a vacuum leak.
Good luck, let us know what you find....