Is the amp for subs or just for the door speakers? If it's pushing the subs, you'll probably be best off just finding a place in your luggage area to mount it. You might even consider mounting it to the back seats if you want to save some luggage space.
the lower gauge wire you run the better depending on how many watts your pushing... runn 0 gauge back to a distro block then 4 gauge to the amp if your amp doesnt take 0 gauge run your Rca cables on the oppositee side you put the power wire on. Also Look for a place where wires go through the firewall already instead of making a new hole.. also you could mount the amp right to the sub box