Do you have the right board selected in the Marlin firmware?
There are some that use the D8 heatbed pins for a fan.
#define BOARD_RAMPS_13_EFB 33 // RAMPS 1.3 / 1.4 (Power outputs: Extruder, Fan, Bed)
#define BOARD_RAMPS_13_EEB 34 // RAMPS 1.3 / 1.4 (Power outputs: Extruder0, Extruder1, Bed)
#define BOARD_RAMPS_13_EFF 35 // RAMPS 1.3 / 1.4 (Power outputs: Extruder, Fan, Fan)
#define BOARD_RAMPS_13_EEF 36 // RAMPS 1.3 / 1.4 (Power outputs: Extruder0, Extruder1, Fan)
An external controller usually gets the input switching voltage from the D8 pins and then turns on the high current 12V side of the switch to power the bed, thus taking the 9amp load off D8
Steve