dual battery

Hey guys, I have set up a dual battery system, I have it all set except one thing. I need a place to attach the solenoid's wire - the one that tells it when the ignition is on or off. Which fuse can I attach this to, or which wire, I haven't had any luck figuring this out or finding info on it...

I live in the desert and it's hot and I haven't noticed a lot of ice cream trucks this summer, so I thought maybe I could do something about that. My friends are all excited about the prospect of buying delicious ice cream from me so I'm actually doing this. I've made enough progress that I...