Finally I have a chance to post! My sleeping platform is modeled on TardisDeli's, but with some differences. Built for comfort and storage space on a 33-day
trip to Inuvik.
It folds to 1/2 length, with the sides folding too. The second row seats are slid all the way back. In daytime they can be faced forward and seat two. Here it is in folded position (with one "leg" open)

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The photo below is with most of it in sleeping position (only one "leg" is still folded). Note that the top folds down at both the tailgate and most of it folds forward. The seats, slid all the way back and facing back, are folded down, touching the platform. Headrests can stay on. Note that the platform does not go full width. It is 48" wide, with a corner cut out to clear the tailgate corner. The platform sits tight against the tailgate. The 4" folding edge allows one to reach down to reach the tailgate latch. Our mattress is 41" wide 4" thick foam ("twin" size), so there is room beside the mattress to put a book, flashlight, etc. I also put large sheets of that no-skid rubbery mats under the mattress so it won't slide.

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This shot is from inside looking back, with the platform folded. The missing triangle from the rearmost section hasn't been attached with a removable hinge yet. I didn't need it on the trip, and it isn't a high priority right now.

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Just folding the 3rd row seat flat works quite well, especially if you use an air mattress. The main reason I made the platform was to gain storage space. Under the seat is only 6" high, under the platform is 13". This photo shows what was UNDER the platform:

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The pile in the middle was under the platform between the sides. Went from the back all the way to the 2nd row seats. The boots and other stuff on the sidewalk was under the platform between the side of the platform and the side of the Delica. The stuff on the left (fire extinguisher, etc.) was beside the platform on the passenger side. Only the grey case stuck up above the level of the platform. In this photo you can see the no-skid under the mattress, and see that the mattress is narrower than the platform. During the day, the mattress was folded in half, leaving the 2nd row seats usable. On our Inuvik trip, the platform was never folded, only the mattress. At night our fleece jackets were on the 2nd row seats under the mattress to level it out some. And my computer brief case (lawyer-type hard rectangular case) lived between the 2nd row seats, nicely filling that gap.
Because the platform plus seats is longer than a 6' mattress, we could put our clothes on the end of the seats. We slept with our heads at the tailgate end, with mosquito netting covering only the mattress. Attached to the upper sidewalls with magnets, and the sides tucked under the mattress. Very effective.
That's my two cents worth.