Hi,
If you peel back the carpet in the front there should be either redundant holes with plugs in them in the firewall. If you haven't got any of these you can usually slide the cable through an existing cable run by enlarging the hole in the rubber plug or if there's room making a new hole in the rubber plug. Remember to keep the speaker and power cables apart if possible in the car to reduce induced noise.
Cheers now
Ken Chapman, April 2007