There's a grommet back up behind the brake that I used. It was a little tricky to get it back in just right, but it's been almost 2 years and no leaks at all. Just make sure to route your wire appropriately in the engine bay and use wire loom where it could be damaged by heat or abrasion.
---------- Post added at 10:16 PM ---------- Previous post was at 10:14 PM ----------
If you're just looking to tap power for something inside the cab, you can use an add-a-circuit. It's easier and safer.
Here are a few pictures from the mod I did a while back. Should give you a pretty good idea of where I went. I used black loom through the first of the two gaskets, so the wire isn't bare, even though it looks like it.
To be honest, I have no idea! I used a pen flashlight through the hole with a friend in the cabin and then fished the wire down to him. All I remember is it wasn't that hard with 2 people once the light shined through. I also remember routing the wire in the engine bay was really convenient.