AFTERMARKET VOLT GAUGE INSTALL

There are 2 switched 12 volt wires at the OEM instrument panel connector up until around 1972. One wire is blue with white tracer. It supplys 12 volts to a gauge voltage limiter or regulator but also branches at the ignition switch. That other branch goes through the bulkhead connector to supply 12 volts to charging system and everything else under to hood that requires 12 volts. Look at the ign' switch connector to find 2 of blue w/ white in the same female terminal.
Now back to the inst' panel connector. The 2nd switched 12-volt wire supplys 12 volts to the brake warning lamp. If my memory serves, this wire has a black casing. I would need to refer to a FSM to know if this wire is fused or not but I know for sure the blue w/ white tracer is not fused. I would consider the black wire when powering a volts meter even if a inline fuse is req'd.
There are a few things to not be so fond of in the 67 harnesses. For example... since the horn relay is mounted right at the horn(s) on the core support, they jumped from closest 12 volts source (your alternator) directly across midair over to said relay.
The horn relay was relocated, and other oddities were sorted/improved over time but for 67, even the reverse lamps were not fused.
An owner could back over something left in the driveway (thank God there wasn't a child on that tricycle) blow a fusible link that kills entire car and need a ride or walk to work. Good luck with yours