2 options:
You can tap into the factory wiring harness behind the glove box and let the factory wires do the work. They are 16ga and will work fine...run your own wire up to the factory harness and solder it.
If you want to run your own speaker wire, use the transmission tunnel to bring the wires to the rear doors. You can run the speaker cable under the front seats. The streams will cross at the the driver's side rear door only...tape the wire to the inside of the trim panel to keep it away from the + power cable. The wire will be close but it should not touch.
