Rob the Ohme chargers are sealed from factory. I do remember seeing somewhere that they do the tethered in a 8m as well but I might have made that up. Best bet is to speak to the installation company and or MB and see if they can swap you over the the Untethered Charger and then like Kezo said try and get them to send you the 8m cable, or like Phaedra said buy your own.
or perhaps the installer can move the unit a few inches to give you a bit of slack to make it work?