The clear plastic part of my sppedo unit has a bit of a crack in it and I was thinking about changing it for another one. Has anyone done this before? I'd expect that the unit doesn't store the mileage but just displays it from whatever control unit does store it (i.e. you just plug a new unit in and it displays the correct mileage for the car) but can anyone confirm this? Also how hard is it to extract the speedo unit from the dash?