How do you run a sub wire through the firewall of a 95 maxima?

Look for an exhisting hole that you can piggy back on. If there is no hole, then you will have to drill one. Be sure and use extreme caution and do not drill into something or another wire. After drilling the hole, be sure and install a rubber grommet around the wire to prevent the wire from shorting out on the sharp metal, and to seal the hole.