On the low current usage stuff like a radio or GPS etc. a fuse tap will probably be all you need, below are a few examples by PICO.
Larger stuff most like to use relays, I found a nice new relay harness, also by PICO. I love them because they are color coded for ease marking on your schematic.
Fuse tap with leg and seconary fuse.
Fuse Tap
Fust Tap Mounted
Relay Wiring Harness
