Yes, that is the one. 3 red cables, one of them with a white stripe and the brown ground.
The fan switch has two positions, I do not think switch has a builtin resistor so the relay must somewhat regulate speed then, builting resistor?
In diagram SL33 or 35 the later style regulation is three step with resistor ( the sliding handle ) but L57/59 does not mention anything.
In the later style diagrams there is a green/white for rear foglights, perhaps the 911L model had this as well, I can always check..
My car did have (aftermarket?) condenser and some AC lines, and it does not have a fresh air fan but that was maybe not std in 1968?
Just found a link with description, maybe worth looking into:
http://forums.pelicanparts.com/porsche-911-technical-forum/81633-help-66-early-fresh-air-blower-wiring.html