BraunAbility is the mobility solutions company that retrofitted NYC taxis with wheelchair accessibility (as seen above with the Nissan NV200). They also devise modification that wheelchair users can purchase for their own cars. One particularly innovative example is their Chair Topper, which utilizes typically unused roof space and thereby frees up storage/passenger space in your car:

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Stored within the rooftop shell, the wheelchair is completely sealed from the elements. The winch can haul a maximum wheelchair weight of 44 pounds, and the modification adds 21.5" in height to your vehicle. It keeps the interior of your car clean (if you've been rolling through muddy puddles, for instance) and works well in tight parking situations.

You can learn more about the Chair Topper, and/or find a dealer, here.