[goldgunner]

I'm sure I'm missing something - or not understanding - but this drawing, as well as the Bentley, seems to me to be showing a single stage switch (for a single stage relay and heating).

But, if I'm wrong and this drawing is right for wiring this switch, AND the numbers correspond to the numbers on the switch (with the exception of the '0' on the drawing above, versus '3' on the swtich), then I could assume (scary) that my:
Terminal # 1 = Black (+ power)
Terminal # 2 = Blue/Black
Terminal # 3 / # 0 = Brown (Ground)
Terminal # 4 = Grey/Green with a Black wire to the Relay

What I'm trying to use is:
Porsche shop: Current Flow Diagram, 911SC, USA, Model 78
Porsche shop: Current Flow Diagram, 911SC, USA, Model 81, Part III
Bentley 911SC Elec Wiring Diagrams - Defogger
and your schematics,
All to start the installation of all dash switches and gauges into a brand new wiring harness.