Folks who suggest using vehicle wiring, and/or cutting holes into existing grommets are off on the wrong foot. If they cared to read the vehicle manufacturer's suggestions, they'd know this is NOT the correct procedure.
There is an easy way through every firewall, even if that requires a hole. An NMO saw like those sold by Antenex is an ideal tool for such things. This said, some vehicles make it easy to wire through. Most Hondas have a hole just for this purpose. Some, like Toyota, have an extra teat on the main wiring harness. It isn't necessary to punch, cut, or probe if you know where to look.
If you're afraid to do the work, get a local sound or two way shop to do it for you.
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