I know this thread is years old, but I just wanted to let everyone know that the suggestion above worked great for me! I was getting ready to drill a hole in the firewall metal when I figured that I'd search this site to see if anyone found anything better, and this did the job. Of course, always verify and check your work with anything you do to your car, as your car may be different than mine.
Just follow the hood release cable to where it runs out of the passenger compartment, on the side above the driver's kick panel. And right near there, at least in my 2015 LX Coupe, there is a blank rubber plug, about 1 inch in diameter. I just popped it off, cut a hole in the rubber plug, then ran the wires through the hole in the rubber plug and then through the factory hole in the metal, to under the hood. Then, I carefully pulled all the wires through from under the hood, got rid of the slack under the dash, and then popped the rubber plug back into place. I tied wires inside the car so that they slope down toward the plug to help prevent water from coming in, but of course, I still need to properly seal the opening in the rubber plug.
It was a little tricky to access the wires on the other side of the metal (it would've been easier if I had the proper tool -- a wire grabber that I used to have when I was a car stereo installer), but it still wasn't bad. They come up in the space between the fender body panel and other under-hood structures, and there is a piece of hard foam in that area to work around, as well.
I'm steadily working on a variety of projects for this car, some of which may not be implemented for months, but I want to get as many wires in place as possible so I don't have to keep "reinventing the wheel" and taking things apart over and over if I don't have to. I've got a dozen wires now running under the hood, and the possible uses for these wires are, in no particular order...
1. An under-hood/engine temperature sensor (not hooking to factory sensors -- this would just be a thermistor mounted to the outside of a radiator hose)
2. An accessory power feed for any project that may need accessory power that is under the hood (including the above-mentioned thermistor)
3. A fused 12 volt battery feed that goes inside the car
4. Knight Rider lights
5. Fog lights
6. Marker lights (meant to be able to be turned on when the car is off to illuminate the vehicle corners to make it visible at night when parked along a curb)
7. Underbody lights ("neons", but will actually be LED's)
8. An alarm siren (not planning to add an extra alarm right away, but in the future, especially if I do exterior mods, I may want something with an impact sensor)
9. A tachometer wire, in case I want to add an auxiliary digital tach in the future (using an inductive sensor and not tied into to the vehicle's engine wiring)
10. Photocell light sensors mounted by the headlights to go to indicators that let me know if there is a burned out bulb.
11. White LED daytime running lights