Finding your way through the firewall

Between the engine and the interior of the vehicle there is a steel panel that protects the passengers from the noise and fumes of the engine. This panel is called the firewall and as the name implies it also protects the passengers from danger in the event of an engine fire. At some point you will need to find a way to get a few wires through the firewall to be attached to the black control box which will be installed on the dashboard. This step can really be tough and there are only a few tips I can give you to make this process easier.

Tip number one put a wire hanger in your tool box and never do an install without it! I almost never get a single wire through a firewall without a bent up wire hanger, not saying it can't be done but it's crazy not to have one. Firewalls are not all created equal either. Some are several layers thick with space and "stuff" between them. Some of that stuff can cost an installer a lot of money if it gets drilled, remember that. Make sure you know where you are going with a drill bit. A wire hanger on the other hand is much safer if you remember this simple rule. If it doesn't slide though nice and easy don't force it! Forcing stuff often cost a lot of money.
Next tip, follow the cars wiring harness to determine where it goes from the engine compartment to the interior. Many cars have a rubber boot where the wiring harness passes through. In those cases your job just got easier. In the graphic above the blue green lines represent a typical wiring harness in many vehicles. The photo below is a typical wiring harness that you will see in many cars The Photo is helpful in that it shows that "all roads lead to Rome" so to speak. once you locate the main trunk it will eventually lead to some kind of boot or bracket that passes through the firewall.The above photo shows the main trunk going through the white firewall with a hard plastic bracket. Most likely in this case the installer would need to drill a hole to get through. In some vehicles the wiring harness goes into the interior in several locations. In those cases I try get as close to where the car owner wants the control box. That is usually my goal when I have to drill the firewall, also get close to the box. In cases with the rubber boot you can poke the wire hanger through the rubber and pull the wires where you need them.