I did not go into the Fuse Box.
I went under the dash and probed the wires in the harnesses that led to the Pedal Position Switch and another switch (I don't remember which as it was several years ago) to obtain "continuous" 12V and "switched" 12V.
I use these taps to power numerous devices (through individual fuses) such as my Garmin GPS, Radar Detector, TPMS, Rear View Camera Monitor (on the back of the 5th Wheel), and Dash Cam.
That way I did not have to go through the firewall with any wires, or into the Fuse Box.
RodeoGeorge