There is a market for the factory subs, look on ebay for an example. If you would re-use the sub in another location, I'd surely be interested in the remaining enclosure.
I see several on ebay that are new and they aren't cheep. Most are asking between $299 and up to $449 (not that they will get it) Mine has a couple years of use, and not as pristine, but would be a deal for someone looking for an inexpensive and easy upgrade. I figure it's worth $100.
I got a price from my local dealer on the replacement panel for the rear (non-sub) and with a local forum discount, it's $115 ($154 retail)
I did some more investigation, and I thought there would be enough room to mount the Kicker Hideaway under the rear seat, but it doesn't look as if there will be enough clearance when the seat is folded.
So the primary option for me looks to be under the driver's seat. I've read comments on other forums about that location providing a "butt massage" which is funny and all, but curious if it would get annoying after a while. I suppose much of it depends on how high you have the gain turned up.
Does anyone here have a sub mounted under the driver's seat who can provide some first hand (or first butt) experience?