have you tried taking her for a quick run before getting in the car. or maybe puting her in the boot of the car. maybe if she cant see things wizzing by she might not be sick. have you given her a chew of a kong in the car to destract her.
this is a bit daft but have you tried driving just around the block, if you see any signe of her posible being sick, pull up somewhere and see what happens, drive off again.
i think its just going to have to be more car journys and longer ones till she gets over it. sorry