On my civic, I had trouble finding it too. I found one behind the accelerator pedal. Look for rubber pads on the firewall, peel it off, you might see a piece of foam or something on the other side, that will just push back, and you can slide the wire through.

I've also been told to look around the master cylinder for some holes. If all else fails, you might check the area where the headlight wires and such go through.