Marlin (on RUMBA board) switch extruder fan automatically
The RUMBA board has two fan outputs. The primary fan output is switched by pin 7, the secondary one by pin 8. The reason your fan is not working when you set #define EXTRUDER_0_AUTO_FAN_PIN 8 is because it's switching the secondary fan. If you switch the wires over to the secondary fan output it will work correctly.

If you do want to use the primary fan output then you can achieve this by setting #define EXTRUDER_0_AUTO_FAN_PIN 7 but you will need to search for the #define FAN 8 in the appropriate pins.h file and change 8 to something else (-1 or 7, for example).

I would recommend the first solution, because you should have a controllable print cooling fan, and an automatic extruder cooling fan.