Autometer A/F Gauge
For getting through the firewall, it's usually easiest to go through the factory PCM grommet on the passenger side of the car. Unbolt the PCM from under the hood and lift out, you should see where the PCM wiring runs through the firewall. Pull the grommet out, feed a wire though with the factory stuff, and reinstall. You'll need to pull the trim from the underside of the dash on the passenger floorboard to grab the wire, but that's about it.
If you need some help getting the wire through it, I've used a wire coathanger run through the hole to either pull small wires through, OR to stretch the grommet and insert a larger cable (like the 1/0 power cable for my amps).
Good luck!

