While not diagnosed with fibro, I, too, have difficulty in a vehicle for long rides and even driving. I either bring a regular pillow or a sleeper pillow that goes around your neck. I sometimes put my feet on the dash as well, but I know it is dangerous. Maybe moving the seat as far back as possible and stacking some pillows or a stool under the dash? Or if it's a long car ride and it's just you two, sit in the back with your legs across the seats. Bring a pillow to lean somewhat against the door.