I find just simply stretching my arms as far as possible (while keeping control of the vehicle) will make the middle of the back more exposed to the seat. Then you can just rub you back up against the chair - hard enough so that it can somewhat satisfy the itchiness.
Note, this is useless if you're in a leather seated car.
This may be uncomfortable sometimes.
Lay a jacket on your seat. This way when you get the urge to scratch you can rub your back on the jacket. The ridges and texture will help alleviate the itching.
If you have a steady hand while driving you could try using a back scratcher. These are pretty inexpensive.