If you’ve ever put an amplifier in your boot the process will be much the same although you’ll be wanting some much fatter cables.
Find a suitable route that will give as much protection to the cables as possible, through the cabin and under the carpet if you can or outside and underneath if you can’t, probably best you run it in some flexible conduit for extra protection if you’re going outside. You’ll need to cut a hole in the bodywork into the boot or into the dashboard cavity, use a compression gland or two to stop air/water/gunk from working its way in. Unbolt the battery connectors and find a suitably large crimp or connector to join or solder and heat shrink (if you have the tools) and connect your extension cables. Reattach battery connectors at the boot end and voila!
Can’t say I’d recommend it though, would you mind if I asked why you’d like to do this?
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