Find Keyed 12v Source

Am adding 2 way radio to a 2001 Expedition. Where can I find a keyed 12v source to tap into? Everything seems to be sealed (or hidden). Don't want it to be half ass.

Look at a wiring diagram for your vehicle. See if you have an unused fuse location. Otherwise use the radio circuit and splice into it. The interior fuse box location may provide the easiest access.