Separate blowers for heat & air?

I was working in my 1976 300D w115 the other night, decided to run some heat. No heat. Sympoms as follows:

AC will turn on blower
Blows only cold air
No blower with AC off
No air to floor vents

AC Vacuum switch tested
Vacuum actuators function properly under dash
Change over switch functional (metered out)
Flused heater core, all lines hot.
Blower switch has 3 speeds W/AC on, inop w/AC off
checked fuses and Voltage from ignition switch
checked grounds
performed search for similar problems

The electrical diagram shows two blower motors, one for AC and one for heat, I think? Are there actually a second set of blowers somewhere in the big black box? Or do they refer to them by thier function at the time. I would appreciate any help before I tear into this mess.