Caring for a Three Legged Dog or Cat

I've a Mazda CX-7 SUV since I fostered then adopted my tripod, "Roo", who's now a 70 pound 2-year old certified therapy dog.  He's never been able to negotiate his way into a the floor of a backseat on any vehicle, and of course couldn't jump into the back of the SUV (he's missing his back leg and has severe bowing of his remaining back leg, hip displasia and arthritis in his hip from losing his leg via a car accident at just 2 months old).  Unfortunately, I was recently diagnosed w/disc problems in my lower back and am told "no bending, twisting or lifting", which leaves us in a dilemma and unable to transport Roo to his weekly therapy dog visits to our local children's hospital and ALF.

When he was a baby, we trained him to use a ramp, however it would never stay in place, shifting, sliding from the bottom, then one day collapsing w/him on it.  He's a smart boy, so there's no way he's getting on that thing again! (even though we tried to rig it by drilling holes in it and securing it w/bungee cords to the vehicle).  We purchased and tried a variety of portable steps, all of which toppled repeatedly when my awkward tripod used them, so they've been returned to the stores.

SO, we were wondering if anybody else has a back-limb amputee who's able to easily get in/out of their vehicle, so we can get a different car.  We're considering the Chevy HHR or mini-van, but really have no idea.

Vehicle suggestions, anyone?  Roo really misses his therapy visits, and we know the patients and their families miss 'the 3-legged therapy dog'.