I wanted to run battery wires into the cab. You see, my truck does not like the cold temps. I got one of those jump starters but I hated to go outside and pop the hood to hook up the jump starter. So I mounted battery posts inside the cab.
Now to the process. i popped the cener cover off and if you look just below the radio, you will see the grey firewall cover. There is only a few inches of firewall visible. This will be on the right side below the radio. After opening up the cover a bit you'll see the painted firewall. Drill there! You will need a long extension for your drill, like 20" or so. It's a pain to get the hole started but it's well worth the time My pictures show the location along with a long zip-tie thru the hole.
The reason I drilled there instead of using existing plugs is that the jumper cables were too long to be effective.
My only regret with these photos is that i used a black zip-tie, sorry.

You can see the zip-tie in the center going thru the new hole.

This is a shot of the area i was working in

You will need a long drill bit. I had to use a few taped together. This was a low-budget operation.

Closeup view in the engin bay with the zip-tie sticking thru the firewall.

This is the engin bay area where my wires will come thru. See the closer image to see the zip-tie.

I mounted the remote battery posts on my dash. One on either side of the cigarette lighter. They are not in the way of anything and I don't have to get out of the truck to jump it.