Steam cleaners are great for interiors. The best technique I've seen is to use Autoglym interior shampoo, microfibre cloths and a steam clearner. The steps are as follows:

1) Spray shampoo onto the seats.
2) Lay microfibre on area of seat.
3) Aim steam at microfibre when it is lying on the seat. The steam will draw the dirt from the seat, onto the microfibre.
4) Repeat steps 2 & 3 until the whole seat is done.