I'm installing a subwoofer under the passenger seat (BOSS with built-in subwoofer). I cannot figure out where to route the power cable to get it to the battery without having to drill a new hole in the firewall. Any suggestions? Thanks.

On the left side of the firewall,between the brake booster and the left inside fenderwell there is a very large rubber grommet. The main wire harness for the truck passes through it. Cut a small hole on the inside part of the grommet, on the left side of the wire harness as you are looking at the grommet from under the hood. The grommet is thick and you need a sharp blade. If you cut an X in the grommet you can use a short piece of stiff wire, like a cut off part of a coat hanger and tape the power wire to one end of the stiff wire and push it through the hole you cut in the grommet then pull it through easily from inside the cab. Bend the coat hanger a little so it kind of points down as you are pushing it through the grommet. It also helps if you grease up the taped area with a little Vaseline or wheel bearing grease so it slides through the grommet easier. Be sure to install a fuse holder and fuse within 6 inches of your connection at the battery positive post, to avoid any fire hazards.

Jimmy

when i ran the wires for my kc switch (it was like 3 wires) i just put them through the same hole that wiring harness was already goin through, worked fine

Yeah I agree with everyone else I had 8 gauge running threw mine no problem pushed right in. You might have to take a long screw driver and push it threw first to get the hole loosened up.

That's what she said.

I also used an existing grommet but I used the hood release grommet.

same here

Haha, I ran mine under the truck and up through the 4x4 shifter.