It goes to the coil pack.
Look at your engine and follow the spark leads back to their origins and it will lead you to a black round box attached to the right side of the engine bay this is where you need to tap into for your rev signal.
There will be 2 small wire there similar in size to the green one you have from your tach, splice the tach wire into the blue wire on the coil pack and the gauge should now work.