I have one of these cut out switches to fit. It is half for convenience (because when I am doing E.g welding or work on the electrics I won't have to remove the battery terminal) and half because I am adding a lot of wiring to my truck and even though I am doing it as carefully as I can, it still could fail.
I won't connect the isolater into the starter cable though as the starter has a nice big solenoid as an isolator.
The isolator switches are only rated at 100A continuous and 500A for 5 seconds. My neighbour has one in his kit car in the starter cable and it gets very hot if you keep cranking the car over trying to get it started. Bypass it and the car cranks over much better.
I read the very sad story of what happened to Byrons truck, although my experience wasn't anywhere near as bad, I had a fire on a Peugeot 306, I had a minor crash in the snow and the next day, when the snow had melted some wiring in the bumper had shorted out and gone on fire. It was about 6.30 am and my neighbour had knocked on my door and told me, I tried to tip washing up bowls of water on it because I didn't realise at first it was electrical, then realised but I didn't have any tools to disconnect the battery they were all at work. So had to use some old pliers to break the terminal off. Luckily the fire didn't catch but it was a lesson learnt for me - when there is a fire you can't think straight, and it may not happen at a convinient time!
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£16 posted from rally design. I will put the T pull behind the rubber flap in the inner wing. If I want to use it for security I will put a small fuse holder bypass on it to save the clock time etc.