It is the white cloth covered wire with purple & orange tracers through it that goes from the fuse panel to the coil. If your harness has not been modified it will also be attached to the yellow wire going down to the starter. It is show in the AIM under the engine compartment wiring.......RatPack.........
There are two primary circuit wires from the ignition switch that connect to the + terminal of the coil; one wire for applying a full 12 volts for starting and a second "resistance" wire which reduces the voltage to the primary side of the coil when the engine is running. The wires to the distributor and the tach lead (if you have a tach) connect to the negative (-) terminal of the coil.
When the ignition switch is in the start position, the ballast resistance (or resistance wire) is not in the circuit, and a full 12 volts is applied to the primary winding of the coil for starting. After the engine starts and the ignition switch is in the "on" or "run" position, the ballast resistance wire is now in the circuit and the other wire is not. The ballast resistance drops the voltage from 12 to somewhere around 7-9 volts, enough for running while saving wear by reducing arcing across the points in the distributor.
Attached pic is typical. It shows a ballast resistor in the "start" circuit. Sometimes GM used a ballast resistor and in other applications GM used a special resistance wire of a given length to create the required resistance to drop the voltage from 12 to 9 or so. Not sure of other years, but my '69 RS has the resistance wire which is easily identified by the cloth like material that covers the OD of the wire as described above.
For my ride, I converted the orignal distributor to electronic via the Pertronix unit - so I didn't have to worry about egine to firewall clearance issues. Some previous posts on this forum have discussed various HEI distributors that were too big to fit without using the "beat to fit, paint to match" approach on the firewall to get enough clearance. Just FYI.
