BMW specialists tested our existing unit and discovered 3 faults, the replacement charge was over £500 inc VAT and Labour. Do we risk a used unit and fit it ourselves ? These suppliers stated "Tested" so we gave it a shot, pulled out the rear seats ourselves and fitted the used unit, it's a "Plug and play" no user serviceable parts. We needed a few specialist tools, but gave it a go, it took about 1 hour to do the entire job, it worked. These suppliers were true to their word. about £460 saved for an hours work. Brilliant.