I am in the process of readying my 2016 C-Max SEL for towing behind my RV. I am adding a Roadmaster Brakemaster supplemental braking system as well as a switch to the brake pedal and running-, turn- and stop-lights operated from the RV. This requires six wires and an air hose to pass through the firewall.
After searching for what seemed like forever I found two ready-made holes sealed with plastic covers in the vertical portion of the driver's firewall.
Just to the left of the grommet through which the steering column passes you'll find a threaded stud which is used to secure the carpeting. Pull back the carpeting and slit the felt carpet pad with a razor just to the left of the stud. Make the slit vertical, four or five inches long, so you can get your hand behind it.
Put your hand behind the felt, about an inch to the left of the stud and feel your way up about three inches. You should encounter a rubber cap inserted into a 3/4" hole through the firewall. Use this hole if you wish but understand that there are things in the way on the engine side and fishing a wire is difficult.
The second hole is about three inches above the first one and maybe an inch or two to the left. This one is like a Christmas present because it measures 2.5" x 3.25" and is a straight shot into the engine compartment with nothing in the way.
Even though you can't see up there unless you're a small person, the cover is easily pried off with your fingers.
I drilled a 1/2" hole in the cover for my wires to pass through. I was originally going to use flexible wire loom to protect the wires on the engine side until I noticed that, with the aid of a flashlight, I could see the hole from the engine side.
I stuck a 1/2" Pex plastic pipe from the front of the engine compartment down through the big hole in the firewall and inserted it into the hole I had drilled in the plastic cover. I then replaced the cover in the firewall and secured the other end of the pipe with plastic wire ties to make a rigid structure. This provided me with a protected wire chase from the passenger compartment to the front of the engine compartment next to the electrical center.
The first picture shows the big hole cover with a small piece of Pex tubing inserted into it as a test. The second picture shows the engine end of the Pex pipe after I had threaded the wires and air line through it.
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