To best of my "limited" knowledge a 89 300ce is basically a 124 chassis and will have the same ACC controls as my 94 e320 Wagon.

If so, with temp dial in hot range and [up & down] button depressed, heat comes out of ALL vents.

Does heat come out of floor vents where center console meets dash?

If heat is only coming out of defrost, it's in default mode. Vac controller is accessed by removing glove box. Vac is supplied through check valve mounted behind master cylinder. This vac feed line routes under heater box and battery. Line often cracks and looses vac pressure causing system to go into default.