Here is the completed hole.
To protect the wires we ran a grommet in the hole to prevent the metal edges from chafing the wires.
Here is the grommet in the hole ready for wires.
To thread the wires through the firewall Vince used a shoelace to first get through the hold and down into the interior of the car.
The gauge wires are pulled through the cowl.
The shoelace was used to pull the gauge wire through the cowl, then an existing hole through the firewall was used to get the wires into the engine compartment and down to the sensors.