When Shelley was a puppy, I built a ramp out of a piece of plywood, with cleats to stiffen it and to give her a foothold. It had a couple of blocks to hook onto the top of the rear bumper of the Expedition. I made it wide enough that it didn't feel narrow to her (a little wider than their natural stance is a good idea).
It worked fine. Of course she can now jump in without any intervention whatever, but that's because she's only 3 and skinny as a rail (for a Lab).