The basis for the plan is a come-along, a manual cable winch capable of lifing 1+ tons.
Now, they come all the way up to the 5 ton variety, but for this kind of job, you want the minimum strength because you don't weigh a ton, (and certainly not 5!) and the heavier the capacity, the more times your friend have to push the lever back and forth to move the desired object (in this case you, but come alongs are great for everything, and even just a 1 ton will handle your power chair if you ever need to tow it out of or into something). So find a good a 1 ton comealong. We actually use one as the main tie down in my van because it failed to come with enough tie down points, and when we we trying to use someone's SUV, it caught my powerchair in midair and held it, then successfully pumped it back into the car.
I obviously don't know what your car looks like, but the next piece of the puzzle is a strong pulley point - this is going to involve a smart mechanic with a welder. A construction is only as good as it's weakest part, so your car needs to have a decent frame around where you want to sit (no crumple zones!), and the weld needs to be very tough too, or the sheer mechanical power of the comealong will tear it off. If you have a mechanic that can attach a high quality pulley, that would do the job too, but if you don't mind getting a new comealong every so often, just a really good, strong loop would do the job.
The wire of the comealong would go through the loop, the hook would attach to whatever kind of harness you're comfortable with, and then you would have mechanical advantage pulling you up and into the car, and while I am not advertising the highest comfort rating, with patience you would only need one helper to lever the winch, then come over and get you centered on the seat, then let the winch down and unhook you (you'd keep the harness on for the ride).
I am under the impression that european cars are smaller, so I'm am a little unsure of how to get a long enough space to bring everything to bear, and the angles would be a lot different than our american hog SUV estimates. I don't have any pictures at this time, we didn't think of that unfortunately, but if you gave car specs, my dad LOVES inventing stuff for disabled folk and claims I don't give him enough work now that I have a conversion van. He would definetely work on a more custom design.
