Here's a photo of my front trunk stripped. You can see on the passenger side two rubber grommets in the front trunk leading to the battery compartment. The one farther out you can't really mess with because it's very thick, heat-molded with all the stereo and headlight wires embedded in it. But the "blank" one closer to the center is quite thin and floppy, and you could easily poke a small hole in it for a cable. Remember that the front trunk is waterproof due to these grommets and the weatherstripping (water gets into the battery area), so any holes affect waterproofness.
bare front trunkHowever, I'm not sure if that really helps your application at all. If I read your message correctly, it sounds like you want to run a wire from the front bumper, driver side radiator intake, to the dashboard. Having recently had all this stuff torn apart in order to strip things out, here's how I would try to route the wire:
1. From the sensor, around the radiator duct (or through the duct and around the radiator) and straight back.
2. Behind the wheel well liner, secure it (with zip ties?) to one of the hard lines that runs along the wall towards the rear of the wheel well.
3. At the top rear of the driver side front wheel well is where the windshield washer fluid reservoir feeds through a plastic grommet to the blue filler cap under the hood. (Top right in my photo.) Feed the wire through that grommet. I think there is empty space in it or you can cut a small hole.
4. Now the wire is in the battery compartment just below the driver side windshield. You still need to get it through the "firewall" and to the dash. There are a couple of ways through I can think of. But be careful because these all my necessity create a hole in the waterproof barriers between the battery compartment and the interior.
a) Poke a hole, or route through the edge of, the black plastic cover panel behind the blue filler cap. This cover sits directly over the fuse box and relay panel by the driver's left foot.
b) Drill a new hole in the firewall and route through a new grommet.