The Harmon Kardon kit is just the speakers, not the head unit. It's not usually retro fittable but it is physically possible, needs a new parcel shelf though for the sub port, unless it's a tourer in which case the sub goes on the side wall of the boot space behind a trimmed cover - the way you would tell if it's got the kit or not is on the front door tweaters - the HK ones say Harmon Kardon on them. The normal remedy for this kind of issue would be a reduction in purchase price (ie knock off the value of the HK sound system if bought as an option), but as it's a lease vehicle it's up to the lease company to decide what they do as the owners of the car.