Whenever I run any cables through, I usually drill through the firewall, insert a snap-in grommet, and run the wire through with split loom. The driver's side is ususally a lot easier than the passenger side due to the HVAC system.
On vehicles without a standard trans, I usually drill through the plate where the clutch would mount in a manual trans vehicle.
Just be sure to use some sort of grommet if you drill a hole. After while the edges of the metal will chafe through the insulation and cause a short. And double check wherever you drill on BOTH sides of the firewall to make sure you clear any harnesses, the brake booster, etc.