I've been looking to put a hole in the firewall to run some heavy cable. I needed enough room for a grommet around some polyethylene tubing. The hole needed to be about an 1". I prefer the passenger side for this project.

After poking around a lot, I saw that there is a very small spot above the heater bellows. On my Camper Special, there is a black/white wire going to the B+ connection there. It is in a protected place, out of the path of water spray, and allows a straight shot under the hood spring assembly. It is cool, and almost out of sight. Those who are installing inverters or other heavy electrics might find the spot useful.

I poked out the molded grommet towards the front of the vehicle, and made use of the location. For normal sized wires, a few would go through with clearance.

The hole was too small. There is little room for a drill. A hole-saw would catch in the sound-proofing material and seam-sealer. Broken fingers are a possibility.

An electricians knock-out-punch made short work of the job. But first I had to make the hole large enough for the punch-bolt. A very large and long drill bit might have worked, but I deformed it with a long screwdriver, and a sharpened stake. (Not very workmanlike...) It is way back in there, about 18 inches. Then from the engine side, you can just get the punch die in there with some fiddling.

If you can, plan ahead when your engine is out, heater assembly has been removed, or the front end is off. Removing the hood is a good idea, if not.

It is a hard spot to reach. Now I need to figure out how to get the grommet in the hole!

 

I have found not all grommets are the same softness/hardness to install.

I too have installed wires above the heater box an boy your right, the easiest part is getting the hole drilled.

I have picked up a few 12" long drill bits at the loco flea-markets for just these hard to reach areas.
But my last attempt to install a wire hole grommet, I had to cut one side of it to get it started into the hole while only working blindly with your finger.

I've even once tried some rubber lube an that was not much of a help.

So yes, I've been there do that
I had though of install a inverter of that side but soon gave up that Idea.

I ended up installing a 2000w one in the underside of the rear of my truck
bed. Then, I ran a #10 power cord back up to the passenger side under the hood to an AC plug weather box.

[IMG]https://cimg8.ibsrv.net/gimg/www.ford-trucks.com-
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Since, I'm always working on or around my tailgate is why, I mounted it just forward underside of the tailgate area left side.
Orich

 

Pictures of firewall cable routing. No pictures, didn't happen...

https://www.ford-trucks.com/g/picture/8493675
In Gallery, also.



There is a spot above the passsenger vent for a cable to come through.
I used curved PVC pipe to provide the path through the firewall, and polyethylene tubing for safety.
The B/W wire is the Camper Special auxiliary feed wire, and later was run up the conduit.


Above the passenger vent, rigid PVC goes into the engine compartment. It protects against the sharp edges of the hole, and directs the new cable.
Polyethylene tubing is around the cable, and goes through the rigid material. Normal wires wouldn't need it, wiring loom would be fine.


Rigid PVC must extend past the first bolt. There is room under the hood hinge, but in an accident, it could get pinched.
I chose to run it through an RV circuit breaker, inside a plastic box, a contactor-relay, and a few inches to the battery.

If you run a big cable, think about safety. Keep it away from the gas and brake-lines, carburetor, and pinch-points.
It could turn into an arc-welder situation.

Avoid low-density polyethylene (LDPE) for use near sharp edges, it is soft.
Nylon is best, polypropylene is great, High Density Polyethylene (HDPE) is ok and cheap.
Regular nylon wire loom with the gray stripe is fine for ordinary wires, like 10gauge or smaller.

 

To the left of the heater cable hole, there is a possible second spot.
It should be a second-tier choice, but suitable for firewall cable and wiring routing.
I don't like it as much, as a carburetor fire might overheat the area.
A fire-resistant aviation fitting would be a nice safety feature here.


Behind the engine is a possible second spot for another hole. It is in a very warm area. This is the stock location of the heater cable, but there is room for other wires. A grommet, enlargement, or sheathing is required. (I later lined the hole with polyethylene tubing). I only run small wire here.



Inside the cab, over the heater fan, is a possible second spot for another hole. Look to the right of the existing heater cable. It is somewhat behind the engine, in a very warm area. This is the stock location of the heater cable, but there is room for other wires in that hole, as shown. A grommet or sheathing is required. (I had some foam wrapped around it, shown torn away for this shot, and later lined with polyethylene tubing). An additional hole, or enlargement could be done, as indicated.

The location is hot, but LDPE did not soften in there, so it must experience reasonable temperatures. Avoid LDPE, it starts to soften at 170F and melts about 230F.

 

I did not like the direction this conduit was pointing. Also, it really got jammed in there, unable to rotate just right. So I tried a heat gun on the PVC (gray) conduit, and it softened right up. I pinched it at the end, was able to get it unstuck, and was able to put a custom bend in it. The black polyethylene tubing will melt on "high" setting, so be careful where you place that electric glow. (Will post pictures soon, and edit.)