bhunnu16,
I sometimes get sciatica when I'm a passenger in the car or driving in the car.

I have problems with a number of lumbar discs (L3/L4, L4/L5, L5/S1) and some other problems with my lower spine including a spondylolisthesis.

To stop myself getting the leg pains, I use a lumbar support and also make sure that the seat is raised behind my knees. Some cars can't be adjusted, but if that happens, I put a small support under the area upwards of the knees, so that my knees are bent more.

Automatics are much easier for me to drive too, however, it's the angle of the seat that gives me most grief.

If your pain persists, please see your doctor as sciatica is a sign that all is not well with your spine.