i just did this the other day. i ran a wire through the firewall to the positive battery terminal. the other side i ran to cigarette port. just clip the positive (i believe its white colored) wire thats run to the cigarette port and solder your new wire that has been run from the battery to it. you can leave the preexisting ground intact. the one thing you want to do however is run an inline fuse along the positive wire. and you want to do this close to the battery itself. just make sure you run the right size fuse.