Not trying to sound like a smartass...but you could always put a smaller pillow behind your lower back when you drive. I have had back problems too and I sometimes do this in cars as well as on planes. It really helps.
There is an inflatable bladder for lumbar support made by an aircraft company that is easy to install behind the seat...cost about $50. toykilla found it a few years ago, and I have one installed in the passenger seat. I'll see if I can locate a link for purchase.
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