UHF install on a GU

So, I've recently made the purchase of a GU for the purpose of taking it to the beach as well as offroading. I figured the best addition for my car before heading off on either endeavor would be a UHF since communication seems to be fairly important.

So I've already hunted around the internet on how to install the antennae and am fairly alright with that aspect and the basic cable routing except my main concern is with passing the cables through the engine bay firewall as well as getting power to the main unit. On top of that, since the previous owner of the Patrol had already done some head unit mods and installed a new sound system and player I'm stumped in terms of mounting locations for the main UHF unit.

The patrol I own is an 04 GU wagon with ZD30DDTI engine and manual transmission, so the clutch plate method recommended by others wouldn't work.

I'm a complete amateur in terms of car modifications so I kind of lack any technical know how (please do be patient)