I have severe sciatica. Can you suggest something for driving a car?  

Lumbar support? Is the sciatica from the legs or the lower back? Depending on the cause, one of these may help: 1) a foam cushion for the seat 2) lumbar support 3) physiotherapy 4) cortisone injection? 5) weight loss? 6) medication for sciatica nerve pain? May affect alertness while driving.
Ice & lumbar support. Having had sciatica from a herniated lumbar disc, here is what I found helpful for driving: ice pack to the back before a long drive, ice after the drive, lumbar support pillow, tilting the seat forward but also reclining the back slightly, limit your driving time based on comfort level.
Take Breaks. Frequently. Unfortunately driving the car will aggravate your sciatica. If you can avoid driving for a few weeks, it might help while you heal. Otherwise try not to drive for longer than 30min at a time without taking a break. Suggest seeing a pain/spine specialist to see what options for treatment are good for you.
A chauffeur. Most often with a sciatica due to a disc herniation, sitting is uncomfortable, especially in a car. Sitting places 4x your body weight across the discs of your lower back compared to 2x while standing. However, the longer you are in one position you also add a static load similar to leaving a weight on a stack of newspapers for several minutes v an hour in terms of how compressed the stack gets.