I have a Taurus and found a place that is not too hard to access or have things to get in the way on the bottom left-hand side of the firewall (if you are standing in front of the car looking at the engine). I put a little grommet in there and for good measure, sealed it a little from the inside with silicone sealant. The wires run out the back of the stereo/ac console on the dash and run under the carpet. The hole and everything is hidden under the passenger side floor mat pretty far enough out of reach of feet.
I actually had the hole drilled for me with an impact drill by some guys at a car audio place in exchange for a Monster energy drink

.....cheaper than buying a new bit!!
Just to add; The passenger side of the firewall may be easier because there is less "stuff" in the way such as brake master cylinder, steering mechanism, big wire bundles...etc. The ppassenger side should be pretty clear except maybe for an AC hose and vacuum hoses.