Keeping one or two of these in the car will keep humiditiy down a bit, which will help reduce condensation in the first place:http://www.frost.co.uk/thomar-air-dry-dehumidifier-for-car.html
- just noticed it's the same as Duggers posted. I use them in my cars that live in the garages as it helps stop mould forming on seats/steering wheels etc.
I think Rain-X basically causes beading, so you'd be more likely to end up with water droplets than a thin layer of condensation - I'm not sure that would be any better.
I had a 12V cigarette-lighter powered demister/heater in my air-cooled campervan. It was rubbish, so don't bother with those.