Where can I safely run a cable through my car's firewall?

Dear Car Talk

Dear Car Talk | Dec 01, 1999

Dear Tom and Ray:

I need to run a two-wire, 12-volt cable from my battery to the driver's
compartment where my amateur radio transceiver is located. The local Ford dealer
could not recommend a location to put the cable through the fire wall. But it
must be possible. Can you tell me how to place a small cable through the fire
wall without doing any damage to the vehicle or the cable? -- Dallas

TOM: There are three ways you can do this, Dallas. Actually four, if you count
going to a place that installs car radios and saying, "Here, put this thing in
for me."

RAY: But we know that since you're a ham radio operator, you want to be self-
reliant and do this yourself. So the easiest approach is to find a wire under the
dashboard that already carries 12 volts. You can easily tap off, for example, the
cigarette lighter, which is already fully protected with a fuse.

TOM: The second easiest thing to do -- if you want to run a new wire -- is to use
an existing hole in the firewall. There are already lots of wires coming from the
engine bay into the passenger compartment. And you should be able to sneak an
additional wire among them and not have to drill a new hole.

RAY: If there's no room for an additional pair of wires there -- or if you're
just dying to drill a hole (which is what we suspect), then you can just go ahead
and drill away. As long as you're careful not to, say, drill into the heater
core, you won't do any damage to the vehicle by making another small hole in the
firewall. Just be sure to fit a grommet into the hole before feeding the wires,
so the wires don't chafe and cause a fire.

TOM: And speaking of causing fires, if you do run your own wire, be sure to put a
fuse for it as close to the battery as possible. You want to be sure that if
anything does go wrong, the fuse blows before the passenger compartment goes up
in flames. I know you ham radio guys love responding to fires and emergencies,
but you probably prefer that they not be your own.

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