Why would you want to do the HDS-990 instead of just normal streaming through the factory head unit?
Krabby pointed out some good starting points to read over before you dive into anything.
I'm not sure where you'd be planning on doing a 3 way front stage, but it would definitely take a good bit of fabrication. You might be able to put a midrange in addition to the woofer in the door, but again, a lot of custom door-panel work. Or, place the midrange in the door and create a kick panel for the woofer (which will then negate any ability to hose out the floor or really even get it muddy).
My plan is a 2 way front stage and replace the coax's in the rear for rear fill (same setup as my current and previous vehicle), and experiment with a pair of JBL nano powered subs under the seats (perhaps mount them to the seat frame instead of the floor to retain the hose-ability/draining of the floors. I'd love for someone to do the hard work for me and see how clean or altered the factory headunit's high level outputs are through the volume range; I would assume they are heavily bass-reduced at higher volumes due to the factory speaker rear door speakers size and am planning on a replica of our Sprinter's set up, just adding the rear channels. It has a Kicker KEYLOC (2 in the Grenadier), into a Kicker KEY200.4s for an active front stage (a second for the rear channels in the Grenadier), and then a set of Hertz Cento or Morel Tempo 6.5" 2 way components for the front (and matching 4" coax's in the rear doors) with the tweeters being dash mounted, perhaps in pods in the corners (and then attenuate them because of their direct facing/firing). Of course tons of CLD and then 3M Thinsulate in the doors (I'll also be doing the CLD/3M Thinsulate through the rear quarter panels, CLD/Thinsulate/3M Low-E above the headliner, and CLD/3M Minicell under the floors).
Of course, I'm still about 3-4 months out from having a vehicle, so, who knows what cool new stuff could come out or information be gained between now and then!
