Well, there are lots of ways of doing that. The easiest way is to take out the front left speaker panel, you can then feed the wires from the rear of the CD player to the area of the front left speaker. Then attach with tape the ends of the new speaker wire to the end of a stiff piece of wire (an old coat hangar straightened out) and gently feed the new wires forward and down until they appear at the bottom of the knee bolster (the padded panel below the dash). If they dont appear they will probably have gone behind the footwell carpet so pull the carpet back. Once the new speaker wires have reached this area you will be able to undo the tape, withdraw the stiff piece of wire then pull through the speaker wires. With these pulled all the way through you should then be able to route them under the carpet to the rear.
If you have the tools to take out the knee bolster then routeing is even easier.
To remove knee bolster; remove bellows rubber ahead of gear lever, undo 2 screws underneath. Remove 4 screws, 2 on each side of centre console. Remove ashtray and undo 13mm bolt behind, remove centre console. Remove lower steering column cover, 2 screws behind steering wheel and one large plastic screw underneath. Remove bolster, 2 10mm bolts, 1 on each side under bonnet(hood) between outer and inner wing and finally 13mm bolt in centre of bolster in car. With that removed all will be revealed and access is easy.