If you want to follow the factory wiring path, take the driver's seat out as well as the sill plate and rear seat bottom, then go under the carpet near the existing harness' path. You might be able to fish something through with just that but will probably have to remove the seat top and loosen the shelf, or get in the trunk side and work from there. Only other shortcut I can think of is if you want to route it under the sill trim or edge of the carpet instead of removing the driver's seat. Seat removal is easy but they are heavy. There may not be enough wire to route it in the described location though.
