If you are adding electrical equipment, chances are pretty high you will have to pass electrical lines between the engine and passenger compartment.  A couple of people have drilled holes in the firewall but Mike pointed out a rubber bulkhead that Chevy put for that purpose.

The passenger compartment side of the pass through is in the passenger side footwell.
Pull back the carpet and you will see it.  You might want to remove the plastic cover under the glove compartment for better access.
Close up of rubber bulkhead.
This is a photo of Mike's installation.  I believe those are 8 gauge power lines.
From the outside of the car.  This is the passenger side front wheel.  The blue thing near the arrowhead is the oil filter.
This is the bulkhead.  Note the blue oil filter on the right side for orientation.  I'm not sure how Mike passed his electrical lines through.  If your wires are small enough, you might be able to get them in there without cutting the rubber.  If you do have to cut, be very careful and do not damage the existing wires.  I do not know what they are.