Remove the foot plate, or threshold under the door. You may find a hole already drilled, or you can drill one under there AFTER making certain no other obstructions exist. You can use silicone caulk or
rubber grommets (Radio Shack) to keep water out. Replace the threshold cover and no one is the wiser.
Take the wires up along the frame rails up into the
engine compartment. Use wire ties to keep the wires
neat and out of the way. Make sure you route them away from the engine manifold (hot) or where they may
rub against the wheels. This is easier than trying to
get into the firewall and/or risking drilling into something you'd regret (UH OH!)