I also had Sciatica episode that lasted 2-3 months. Chiro was of no help, yoga and various exercised did nothing for me. I finally decided to pay for quality Physical Therapy, and on my 3rd visit to PT, he stuck his thumb into my butt cheek, held it there for 2-3 minutes, and i haven't had any pain since. its been 3-4 years now.
But if you are set on getting a new car, my GF has 2010 Range Rover, and seat is elevated enough where my hips and knees are inline.
Alternatively, if your car has ample head room, you can put down pillow(s) on the seat to elevate your hips.
I am sorry about the pain, i know how miserable you must feel.