Electrical question, tapping power for relay

I get how all of the basic electrical stuff works. I've already got my CB wired in direct to the battery and it works. I'm currently working on rewiring it (and a 20 watt PA) to a pair of relays so they come on when the truck does.
I understand how the relay works, but what the hell do I use for a tap for the relay signal? What wire/fuse on the truck should I tap? I looked through both fuses boxes and I'm not sure if I just use a add a fuse and to which one?

 

Tap into the cigarette lighter plug. It turns off with the truck and has plenty of capacity for a CB.