Spare keyed fuse
 

I have a 2007 RAM 3500. Looking to add an Aux fuse box with power drawn from the battery through a relay. I hope to use a spare fuse position in the factory fuse box under the hood near the battery. I have a list of what should be spare (unused) fuse positions to power the relay. I have a nice 40 amp relay with a built-in fuse, ATO style for this purpose. I need a spare fuse position that is tied to the run position of the key.

I have not yet opened the back of the hood fuse box, and not sure what I'll find there. I see the color code for all of hood fuse box wires. Are these colors for wires exiting the fuse box? Are there already wires connected to the load side of these spare fuses that extend outside of the fuse box and then terminate? Is there a better way to use one of these spares.

Thanks for any info.