SMC Kit-Where to run wire?

I ran mine through the hole that is already there. There is a rubber grommet near where the stearing column comes through if I remember right. I have just about all my aftermarket wiring going through there. I think it is about 1" - 1.5" in diameter.
You can buy the rubber grommets at an auto store and drill holes where they are needed at an appropriate place in the firewall. For example, since the existing hole/grommet on my car was full of wires and a vacaum hose, I drilled from the inside of the car towards the engine in the area above the accel pedal. Just be sure to zip tie the proposed wires up in the engine compartment and put the plastic wire protecter hose on it for protection and appearance.