That's a tricky one as backs are such a personal thing.
I have found mine is helping my back and an improvement over my previous,
pretty comfy seated, car.
There are inserts and things you could look at - new seats could be expensive
and you might end up with it just the same - or worse!
A good idea would be to consult a Physiotherapist or similar for suggestions...
as ever a good old surf of the internet might help.