You have to drill a hole to run the power wire from the engine compartment to the interior. Be careful not to drill through anything on the other side of the firewall and USE A GROMMET. Another option, if it's small wire, is to remove the driver's side inner fender and poke the wire through the existing factory grommet to get inside. You'd first have to shove a screwdriver through to make the hole (being careful not to hit the factory harness) and route the wire from the inside out after that.
Since it's just one amp, I would use a ring terminal to attach the wire to the battery post. Make sure you install a fuse within 18 inches (or so) of the battery.
Use a ring terminal for the ground wire within 18 inches (or so) of the amp to bare metal.