Booyhoo, With a more set minded dog, IMO you need to think like him - ie to not be moved, to not let frustration or doubt cloud your thoughts. Beagles are single minded and so must you be. Let us know how you get on
Blonde, the lack of power in his back legs may be key, if it is difficult for him then he won't see the point IYSWIM. Again I would try training him to go up things. It may sound stupid but take the ramp and put it flat on the ground in the garden where he has to walk over it every day to get out. Then put a brick or something underneath one end. Command him Up, In or what ever you want to use for the car when he puts his first foot on it then treat him when he gets off the end of the ramp (make sure it is stable!). After a few days add another brick.. and so on. Just make sure that he never has a big drop off the end to jump down, maybe get a cheap table off Freecycle and cut the legs down so he has a platform to go on to to get his treat.
Anyway, the key is, one building the slope slowly and two building the rear muscles up at teh same time - which will happen naturally as you increase the incline.
At the same time you need to make sure the car is not stressing him out. You need to get him to it every day or more, carry out his obedience training by the car so that he is always getting treats near it, get your OH to lift him into the boot and treat him then take him out, work up to being able to groom him in there, treat him and take him out.. give him his bones in the car and drive around the block- anything you can think of to make it a nice place, just like crate training.
Then put the two together, nice bone in front of nose then placed in car, treat shown and command given to go up ramp. He eats his bone and back out, without closing the door even and always get him out well before he wants to get out IYSWIM... and build it up and up from there.
Well that would be how I would try it anyway, hopefully my ideas will give you some thoughts?