There is a certain science to a fuse box. I've wired up a lot of projects over the years to cars, bikes, even a couple of boats. Fuses are the most important element in a circuit aside from the function of the circuit. Fuses must be properly rated to protect the wiring harness and the designated load such as a stereo amplifier or fog lights etc.
If you have a Fluke DVM then I will assume you know how to use it and you should be able to find the slot(s)
that is switched. For example, I used the switched power outlet fuse slot to add my circuit. The power outlet is
fused at 15A. I was adding a 5A fused circuit, so I dropped the outlet fuse down to 10A as it was primarily used for a low current draw phone charger. 10+5 = 15 so I preserved the 15A rating of the fuse slot. Very important not to overload the 'slot'.
One reason for disconnecting the negative cable from the battery is that modern cars utilize negative grounding
with the metal chassis of the car being used as a giant piece of wire. If are you working on an energized circuit
and inadvertently slip with a meter probe or bare wire and touch the metal body or chassis, you can damage stuff because there is a complete circuit allowing current flow. Fuses can blow and it's a hassle to locate them and replace. Easier to just pull the neg cable first.