You do not ground a +12v wire from the battery, that is a direct short. The wire should be connected to the "+" terminal on the battery, go straight into a fuse holder as close to the battery as possible then continue into the passenger compartment. If you pass through the firewall, make sure it is not rubbing on the edge of the steel. If it is put a grommet in the hole to protect it from wearing through. If I were you, I would add a relay to this wire as a second output and control the relay from the factory ignition wire. Then you can hook what ever you want to it (within reason) without worrying about the factory circuit being over taxed. You could also add a heavier gage ground wire if you wish and connect it to a bolt passing through anything steel in the dash frame or firewall. Then you truely have a stand-alone power circuit. That way if you ever have electrical isssues with the vehicle, the dealer should not question any "modifications you have made.