I posted this in the Mazda forums but nobody seems to know but figured it's not specific to my car and is a general question so wondering if anyone here knows? If I connect an add-a-circuit to a fuse which is in use by the car and one which is always on (i.e. the interior lights) then I get a reading from the multimeter of 12.4V from both the fuses in the add-a-circuit. I then connect a 15A fuse to an unused fuse slot (not to be confused with a spare fuse) such as the one next to the power locks (30A) and I'm getting a 12.4V reading, however, if I remove the fuse and connect an add-a-circuit to this same slot instead I still get a reading through the fuse in the add-a-circuit which is for the original OEM circuit but I'm not getting a voltage through the second fuse which is for the new circuit you are adding. I'm wondering if it's because there isn't actually a device in the car trying to draw power from it (seeing as it's an unused slot) but there is 12.4V running through it so it shouldn't matter! Does anyone know why this is happening?