Ignition Power Source

[fiftycars]

I'm installing a new head unit and it is just a replacement with no amp or other added power needs in my 03 Silverado. I'm not using the module to retain the chime and since there's no power line in the harness I have to tap into a source. Rather than running out the firewall into the fuse box for ignition power where can I draw from inside the truck? It looks like there's a 10 amp source (marked "ign") in the fuse panel at the end of the dash but it's not real clear to me from the manual. I've installed a fair amount of stereos, amps, equalizer's etc; but not much in newer stuff and don't want to tap into the wrong thing.