This works. I always put her front feet up on the seat first and then started the nudging but I don't think you have to do that. I gave a consistent command every time, in our case it was "Summer, in". She's now learned it and will happily load into any vehicle I ask her to. She gets excited now when she's near the car and I unlock the doors and she hears the click.
We did the same thing with jumping on the bed. My bed is unusually high, simply because I'm tall and I like it that way. So she had to learn. I have a bad back and lifting forever is simply out of the question. Front feet went up and hubby or I hoisted the back end. This took some weeks but finally she did it herself. Praise praise praise and the crowd went wild! Now... I just snap my fingers, "Summer, up", and we're a done deal.
Edit: I've seen other people toss in things to entice their dog to jump in. It's always been something super yummy, like dehydrated liver or lung.