when wiring directly to your battery, you do need to install an inline fuse and a switch. you'll drain your battery otherwise because you're carrying a constant charge to the scanner if you car is on or off.

when tapping a fuse, you need to locate one that carries a charge when the car is on. all you need to do is split the wire, ground the black wire to the body of the car, run an inline fuse along the 12V, or power wire, and wedge it between the fuse and the fuse holder. it's like making the wire stay in there with the fuse acting as a pin.