Ouchy,
Comfort is hard to find being a driver or a passender.
This is something I'm always trying new things with.

The best thing FOR ME is:
A cervical pillow behind my neck (Bed Bath & Beyond) and a pillow or blanket under the seatbelt as not to press on the brachial plexus.
I use this technique for driving and as a passenger.
If anyone rear ends me I don't want more cervical pain and the seat belt crushing my thoracic area.

Dr. Agnew has also perscribed a 20" McKenzie Cervical roll pillow and place it vertically when sitting.
It rises up your posture when sitting.

As a passenger I do the same but with a pillow under my right arm.
Lots of stops to get my flexibility and circulation going.
Most of the time a small dose of pain meds as not to impair my driving to relax my muscles.