Hey all. I managed to get my Renogy DC DC Charger hooked up and working.
I followed the wiring in the Renogy instructions and used a fuse tap on F42 in the engine bay (rear window wiper, it only comes on when the vehicle is on.
Best part is my solar charge controller will shut off when the DC DC charger is going (although I think I was at around 70-80% before turning the car on - not sure yet if it will shut off if the battery is lower). I think this might be a good thing to keep the batteries from charging too quickly, although if you have a big enough bank maybe not ...
I take no responsibility for anyone copying this setup blowing themselves up. In fact I almost blew myself up when I accidently connected the positive and negative on my battery with my wrench (doh). Never managed to do that until this install. I really really hate the place Ford put the battery in this car.