>What determines the 'Seed'? Is it just how many rallies you
>have done before and how you've finished in them?
Yes. Drivers are awarded seeding points based on their finishes.
>Or does the car affect it?
Within the start order, the faster seeds are further broken down by car class. Open, N4, P4 and Gr2 get placed into the start order before the other production classes in the same seed. Good example of that is Sprongl, Bacon and Dascollas starting last respectively in their seeds.