Getting a new wire into the engine compartment

Run down to Hobby Lobby (or your local equivalent) and grab some K&S Brass tubing. 

The tubes are about a foot long and they have many diameters. You should measure your wire diameter or bring a piece with you to get the smallest one that the wire will slide in smoothly. I have found that the 1/8" works well for 20 gauge wire, 5/32 works for 18 and many 16 gauge wires.

Back at the ranch, file one end of the tube to a point. 

{this is where I insert all the lawyer talk about disconnecting the battery, making sure you aren't going to damage existing wires or components, etc... Basically I will tell you what to do, HOW you actually do it is on you.}

Find an existing rubber grommet, plug, etc on your firewall to feed your new wire through.

Push the sharpened end of the tube through the rubber grommet.

Feed the wire through the tube.  I like to feed from the passenger compartment, then route and connect the engine compartment side.
Once that is done I measure out a sufficient length of wire under the dash to make my connections on that end.

Cut the wire and slide the tube down the wire and out of the grommet. The grommet will close back around the wire. 

Remove the tubing from the wire.

That's it.