If you do that, you might not need an extension cable. Just pull it back through to the radio and then reroute out the other side of the firewall.

You'd be getting into the radio either way, I suspect, since if you do need an extension, you'll likely want to put it between the existing antenna cable (with its nicely angled bracket) and the radio itself (assuming you can remove that angled bracket from the line). There are plenty of extensions available. All pretty cheap.