I found a vacum leak in my car... it is coming from the grey wire which is on the top leftmost... it is sucking air and i also hear the other end of the leak coming from in between the two rear tires some where... what could this be? my car has a rough idle, lose of power, and backfiring... can i fix this myself? could it be a O2 sensor?

here is the pic, ignore the red circle its the grey wire to the right of it