I'm sorry I don't have any direct experience with the XC90 models, but I would suggest that you take a close look at the fuseboxes in the passenger compartment. Check for an unused circuit, perhaps one for the optional high-rent stereo system or heated windshield. These new cars have a lot of (relatively) high-powered electrical devices so are more likely than older cars to have easily accessible places to draw significant power.
When I installed an aftermarket trailer lighting system in our previous XC60, I was able to draw power quick and easy from an unused circuit on the fusebox in the cargo area. Not much power required there, but you get the idea.

73!

-Will
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XC60 / Odyssey