This isn't a folding ramp, but it's very light weight and the bottom half is the perfect height for the Sienna floor, entering through the side door.
http://www.drsfostersmith.com/produc...fm?pcatid=2874 It's filled with solid/hard styrofoam.
I have the middle seat out for the dogs. Back when I needed the ramp, the bottom half easily stored behind the back seat. I then took the top half of the ramp into the house so the dog could get onto the bed.
The Fosters/Smith site has other ramps, but I can't vouch for them.