Fish the wire through the conduit that runs between the door and car. Unfortunately, there is a bracket on the inside and the outside of the car which causes a roadblock. You'll have to pop the bracket out.
Once you have done it once the job is easy, it's just a matter of working without line of site. Once you do it once it's a 20 minute job to fish in 2 pair of 12g wires for the Plates and 8" shaker 500 door subs on each side.