IDK if this helps or not, but I just ran a single 14G wire through the firewall.
I pulled out the top of the carpet a little bit to see what I can see.
I saw black rubber stuff under the carpet.
Then I pulled back the black rubber and finally I was looking at bare metal firewall.
Then to my surprize, plain as could be there was a 3/8inch hole in the firewall, no rubber grommet, no nothing, just a hole, no rust, looked to be completely factory. It comes out just under and to the left of the master brake cyl. I ran my wire, and covered it with loom to help with abrasion.
I'm curious to know if that hole is stock on all trucks, or is was put there by a previous owner.
---------- Post added at 09:10 AM ---------- Previous post was at 09:09 AM ----------
work backwards- run the wire from the inside into the engine bay, you'll almost always find a hole.
I second this, that's how I just did mine last week.