Does it go through a standard firewall breach or does it look like an aftermarket hole? Pretty easy to tell if it was done after it came out of the factory. The breach in the wall would be isolated or have that semi-pro appearance to the hole and grommet.
Are the wires bundled professionally to others as it passes through the firewall? These observations can help determine what the wires were meant for.
'68 428 HO M3 Monster, 4-on-the-floor! Need I say more?