If you are talking about the main fan relay in the fuse box at the upper part of your diagram, it looks to me like it gets the ground from the pressure switch. The only pressure switch I know of is in the A/C system. Unless they are calling the A/C on switch a pressure switch.
Cougar, I can't figure out what the A/C short connector is for either, maybe testing? Doesn't make any sense anyway, since those two lines are shown connected together down below.
Zeky, the diode just isolates the ACC hot power and the power from fuse 13, and would affect the sub fan.
I hate subie diagrams.