thanks guys, yep it's the sub-woofer mounted in the spare wheel, with a juicy 400w amp under the passenger seat, part of the specced Dynaudio package.
the sub sits on top of the spare wheel and uses it as a resonance chamber, so is screwed down from above by the big plastic nut. Obviously if you get a flat and need to remove the space saver spare, then the sub needs to be removed and the yellow connector is a quick release connector, for quick removal of the sub.