Those last guys have given you the best route to go for running your wire. From the engine compartment, find a factory grommeted hole (one the has rubber around it.) Carefully poke a hole in the rubber so that when you stick your wire through, there is rubber all around the wire to protect it from the metal of the firewall. BE SURE TO FUSE THE WIRE AS CLOSE TO THE BATTERY AS POSSIBLE!!
There is no easy way to get it to the back, except under the carpet, close to the door sills. Be careful that the wire is not pinched anywere where it could rub away and make contact with the metal of the car.