1966 ACC position ignition switch question

[Riverstory]

I've read many many threads on this board regarding ACC wiring and still can't seem to find the answer to my question -

My goal is to power on the radio using the ACC position on the ignition switch.

I have non-modified original 283 wiring with cluster gauges including original distributor. I also have a new replacement ignition switch purchased from the Truck Shop.

When I turn on the ignition switch to the ACC position, it powers up the brown wire to the voltage regulator only. As it should. That brown wire is not connected to anything other than the voltage regulator.

I have read and confirmed that when the ignition switch is moved to the "Run" position, the ACC terminal stays energized. I have also read that the points or condenser or coil is energized when the ignition switch is in the ACC position. Which proved to be false.

Can anybody tell me why I should NOT 'splice' into the ACC wire (and fuse protect it) to energize the radio?

Thanks and Happy New Year!