Exactly which rear side are you referring to? Above the rear handles or back at the 3rd row for these stickers? Guessing along the bottom of the 3rd row area?

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EDIT>> took a few measurements... if it's the very bottom of the 3rd row window, there's a BMW Sekurit sticker in the way, right after the dot matrix stuff I'm sure you don't want to stick over. You only have 4in. of clear window beside the BMW Sekurit sticker. If you overlap or place it just above, it's a total of 6in. The lengthiest part of that window (higher up) is 9in. (again within the dot matrix stuff).

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'08 X5 3.0si - Alpine White / Saddle Brown interior
Specs: Sport Pkg, Premium Pkg, Tech Pkg, Comfort Access, Aero Kit, Style 433 staggered 20s on Conti DWS
Mods: Carbon 35 tint, LED angel eyes, GP Thunder 7500k fogs, H&R 20mm/25mm spacers, clear reflectors, gunsmoke-tinted taillights
Coded: Digital speedo, windows/sunroof/tailgate close via keyfob

X5 pics at Flickr