From the interior it would be on the firewall about 12 inches above the accelerator pedal.
It goes thru the fire wall as stated. From the engine compartment side it would look like this.
click for larger picture
Disregard the arrows
I believe on this pictured car it is the black and purple wires coming out the top (stick car). This is not a 68 coronet, but the connector location is similar and depending on how it was plugged into the firewall I guess the connector could be on the top or the bottom.