the yellow wire on a headunit is always hot.
it controls memory functions such as clock, bass, treble, fader, etc...
you can fuse it if you want, but it is not necessary (misspelled, i think). the headunit itself has fuses to protect it.
if you get a wire harness for your explorer (you can buy it at wal-mart) it will connect to the factory hot wire. the install only takes a few minutes.