Problem for control speed of a DC motor with a L298N
You do need the ground wire. The reason it 'works' without it is that enable input effectively then has an aerial connected and is responding in the same was as if you touch the input jack on an amplifier and hear a buzzing sound.

There is a link behind the blue connectors that enables the onboard 5v regulator. I assume you have this fitted, or it wouldn't work with the jumper as you describe.

There is a tutorial here : http://www.instructables.com/id/How-to-use-the-L298-Motor-Driver-Module-Arduino-Tu/

I can't see anything different from what you're doing that would cause it to fail, but perhaps you have a problem with pin 9 from your board - you might have damaged it by misconnection, as certain ways of wiring that schematic could put 12V on the arduino pin.  You could also try using pin 13 : the arduino LED should then be lit in sympathy with the PWM signal and you'll know it's working.