.
FUSE TAPS. There is a slick piece of electrical hardware to make clean connections for new electrical accessories, called the Fuse Tap. It replaces one fuse from the fuse box, and provides the original fuse protection plus separate fuse protection for your new gadgets, all in one swift move. As with any electrical work, it must be installed correctly to be safe.
Fuse Taps are available at most auto parts stores, or on-line. They come in two sizes, for the standard .25" blade automotive fuses, and a smaller version fuse used by some new cars. Match what you will buy with the fuses used by your vehicle.
Turn OFF the vehicle, and all accessories. Pick a heavy-rated fuse in your fuse box. Most fuses are switched by the ignition key, but a few may not be. Remove that fuse. Checking the empty fuse socket for Voltage, at each end, can be done with a test light probe, with the wire end connected to any good ground. Pick a fuse that is switched ON by the ignition key, not one that is powered when the key is OFF. The extra load of a Voltmeter or relay coil is so small, it will be inconsequential to that circuit when running. The black lead of a Voltmeter can be connected to any good ground, but the closer it is (electrically) connected to the battery ground, the more accurate the Voltmeter readings will be, as a result.
Install a Fuse Tap into that socket space. Turn ON the electrical power (there is no need to start the engine). With NO fuses installed in the Fuse Tap, there should be NO power in the Fuse Tap wire. If the Fuse Tap has power in the Tap wire, then the Fuse Tap is installed backwards. Just pull out the Fuse Tap, turn it 180 degrees, and re-install it in the fuse socket. Done correctly, there should be NO power in the Tap wire with the electrical power ON, when there are no fuses in the Fuse Tap. Install a fuse of the correct Amp rating for your new accessory into the TOP fuse socket of the Fuse Tap. Now there should be full power in the Fuse Tap wire. That much is now done well. Turn OFF all of the electrical power.
Install the original heavy fuse into the BOTTOM socket of the Fuse Tap. Turn ON the electrical power, and everything should work normally. If so, turn OFF the electrical power, and proceed to install good wiring from the Fuse Tap wire to your new accessory, or the coil wire on a relay-controlled Power Panel (auxilary Fuse Block).
Some accessories have inline fuses; typically they are the size of a pencil, about one inch (25mm) long, made of glass, with metal end-caps. It is better to use the automotive fuse (with the correct Amp rating for the accessory) in the top socket of the Fuse Tap, and simply eliminate the glass pencil-sized tube fuse altogether. That way, you won't be needing to carry any extra glass fuses, and if a fuse blows out, you will know right where to look for it. In-line fuses are usually just a PITA.
There is an alternative to a Fuse Tap which can be used in small spaces, called a Fuse Blade Tap. It is installed onto the blade of the regular fuse, and it should be crushed to fit on the fuse blade closely. You will need to use a Voltmeter or test-light to be sure you know which end of the fuse socket gets the Fuse Blade Tap, as above. With power ON, there should be NO power at the end of the empty fuse socket where the Fuse Tap Blade will go. If the Fuse Blade Tap is at the wrong end of the fuse socket, then your Tap connection wire would have NO fuse protection. Make sure the Fuse Blade Tap is at the correct end of the existing fuse.
It is a good idea to use an inline fuse as close to the connecting female terminal lug as it can be, reasonably. If you wish, you can retain and use the glass in-line fuse (as may be provided by the new accessory wiring). Just use the Fuse Blade Tap at the correct end of the normal (existing) fuse, and the in-line glass accessory fuse, and you will be good to go. The top of the Fuse Blade Tap is a standard .25" male blade connector, and you can connect any mating .25" female blade terminal to that end. This connector part of the Fuse Blade Tap can be bent over flat across the top of the existing fuse, to fit a connection in small spaces. See pix, attached. HTH
Cheers,
Red