I was wiring up a switch for upcoming project and did not see any pictures/DIY so I figured i'd take a few pictures and explain it.
If you want to have a switch or power something in your car cabin and don't want to tap into the factory housing like me (I like to leave things so I can revert to stock if necessary), then your other option is to tap into either your fuse box or directly to your battery.
You'll need:
Bare Minimum:
Wire Coat Hanger
Electrical Tape
5 feet of wire (make sure you have the correct gauge for your application)
Optional (recommended):
Corrugated flexible plastic tubing
Flashlight
Zipties
Fuse (make sure you have the correct size for your application) - either a fuse tap to connect to an existing fuse in the fuse box or solder an ATC fuse holder w/fuse to your + line between your power source and the device/switch you are powering
As always, do this at your own risk and before doing any work related to electrical, disconnect your battery from your car's factory housing.
To quickly summarize, you have a metal barrier between you and the engine compartment called a firewall, for obvious reasons. There are very few places to get through this firewall without having to drill through metal. The places to get through the firewall are where the factory wiring harness passes through rubber grommets. Basically you need to make a very small hole in the rubber and then pull your wires through.
The rubber grommet is behind your steering column (another behind your glove box but farther away from your battery terminals and fuse boxes). You'll have to lie down on your back under the steering column and look up to reach this. Fortunately you do not have to remove any of the dash board to get to this.
Take the coat hanger and make it straight, then proceed to push it through the rubber grommet. It'll take some force but make sure you are not poking into the factory wiring harness. Notice I poked a hole and have the wires drawn through
above the factory wiring harness, this will make it much easier to pull the wires through from the other side.
Important: There are two layers to this grommet with an inch or two of open space in between. You'll pass through the first layer and there'll be a second layer you have to push the hanger through as well. This second layer is the side of the grommet you can see from the engine compartment.
Once you have the hanger far enough through both layers of the grommet, you should be able go to the engine compartment and pull the hanger from the other side. Before you pull the hanger completely through, make sure you take your wires and tape them to the end of the hanger under the steering wheel. Overlap the wires and the hanger with enough tape so that you know the wires won't slip off but not too bulky that it'll create too much resistance when being pulled through the grommet and possibly make the hole bigger than it needs to be.
Here's where you can expect to see your hanger popping out into the engine compartment.
Here's a closer picture of the rubber grommet in the engine compartment.
This part is optional but put your wire into a nice corrugated flexible plastic tubing to keep it from rubbing against anything.
If you want to do it right, pull the wire through about 2 feet of tubing in the engine and give yourself some extra slack wire hanging out of the tubing. Wrap electrical tape around the end of the tubing and wire closest to the rubber grommet in the engine bay and then go back under the steering wheel and carefully pull the wire back into the cabin until the taped end of the tubing is against the rubber grommet in the engine compartment. Ziptie the corrugated plastic tubing in the engine to the existing wiring harness so that it's secure.
This is the fuse box in the engine compartment of a 2009 Camry SE V6, other models and years may differ. The white wire circled on the right is only hot when the car ignition is activated (ACC/on) and is cold otherwise. The nut and plate circled on the left is always hot like the battery.
On the underside of the fuse box cover is a map of all of the fuses and what they are for.
That's basically how to wire from the cabin to the engine compartment, whatever you hook it up to on either end is up to you.
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