Duet Wiring
Duet 0.8.5
Main wiring
All positive fan pins are connected to Vin.
The pinouts are on the back of the board (V 0.8.5), a KiCAD export shows the back below for reference (this is the view from the bottom of the board, so is mirrored):
There is more detail on the pinouts on the Duet pinout page.
SSR wiring - 5V controlled fan
Duet 0.6
The back silkscreen on the Duet 0.6 contains further information on the pinouts, it is reproduced below Beware of the Bed connector polarity, which is different from the 0.8.5 board