1) Do you want to sit in the back? 2) I want to sit in the front.

If the car has two sets of seats, we can say "in the back/front (seat)" or "in back/up front". If the car has a middle seat, we can also say "in the middle (seat)", but we do not say "in middle".

1) Do you want to sit in the back? 2) I want to sit in the front.