If you had to go through the floorboards I'd say just put a .22 through where you want the hole. With the firewall you can't usually do that. You may be able to go *around* the firewall, out between the door and its frame, up into the fender, then wherever. You have to watch that the wire won't get pinched. You can also look at the sides of the front footwell, pull up the carpet and plastic finish parts, usually there's a lot more unused room there, might be an opening with just a plug in it. If you drill out through this you come out inside the fender, then head forwards.
You may be able to find something mechanical, a pipe or tube that already goes through, and sneak a wire through its hole if there's a little extra room. You can drill through the firewall, it's just sheet metal, but you have to really really check that you have a clear spot so you don't drill in through the back of the master cylinder or something. that spoil your whole day. then you have to cold-weld the hole back up and you can't reach it, but aqfter a couple hours poking around getting epoxy on stuff it'll work OK unless you have to pump too hard, then it'll spray a fine mist of brake fluid up on the exaust manifold and remind you that that was not the spot. It's not like I'd know or anything.
As long as you're running a wire I reccomend running a couple extra in case next spring you want to add something else, then the wire's already there.
You can also follow the fresh air intake. If it carries air in, it can carry a wire out. You'll have to deal with little doors and stuff that open and close it. Probably really tight work where you lay upside down on your back and do it half by touch kind of deal. All my cars have required this proceedure. I think they must find it auto-erotic or something. I never have been able to find an extra pin or plug that already is mounted on the firewall, idle and waiting.