i own a couple of diff. back supports that i place on the seat of the car.
one is by Obusforme; I use the high back one so it supports my neck. It was actually made for bench type seats and is good for those who own trucks... I also have a wooden 2 piece back rest covered in foam and ultrasuede looking fabric that's now sold by Jobri. You can find these and the popular one made by Molier at many of those back support type stores and also online. I don't own a Molier yet but I would if i could afford it. Dr Newkirk worked on it and there's one that allows you to pump air into it for TOS. I trade off the supports and also use a pillow under my arms and behind my neck. I had a lesson by a Feldenkreis worker and PT on driving. I learned to hold the wheel low and turn w/my body. When driving a long distance (not something I could actually do today) I would wear my tens unit. And also do my breathwork while driving. Also hold the wheel loose-no clenching.

i like to shop at sitincomfort.com

Why not share what your doctor recomended w/us? and what you've figured out.