I'd recommend connecting the power line directly to the battery. General install rules are: Connect power to the battery with a fuse within 1 foot of the battery and a fuse within 1 foot of the amp. Ground direct to chassis. Ground strap no longer than 18 inches. Connect ground to bare metal. Add a equal gauge ground strap from battery to chassis.
Tapping in to the fuse box could throw a code. Grounding to the battery will cause ground loop noise. You are drawing more power from the battery, so the battery ground needs to be increased. A seatbelt bolt is often used to ground an amp.
Remote on/off lead is generally connect to the vehicle ignition. This should also have a fuse, typically 0.2- 0.5 amp. The amp will only power on when ignition is on.