Running cable through the firewall is always a difficult job-- on any car.

There should be several places where things go through the firewall, you can usually share one of those openings. You don't necessarily try to get your wire to run through the same cabling trunk, but rather locate the rubber seal on the firewall and cut a small hole in that for your wire (sealing it afterwards with RTV, of course). That's what I did on my Accord.

It's definitely not an easy job. Don't expect it to be a walk in the park.

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Tony Fabris