Ok Alan,
I got the schematic in my hand, (wine glass in the other), and it would appear that the PINK WIRE powers the ignition coil and the BROWN WIRE goes to the fuse block where ORANGE to Engine Fan, BROWN to AC/HEAT, TAN to accessory connector,as well as TAN to the Radio all branch off.
The BROWN from the Alternator runs DOTED LINE to the fuse block where it meets with the BROWN from the Ignition Switch.
I would place your GREEN LIGHT in series with the BROWN wire from the alternator without any diode, as the PINK wire feeds the coil and is not conected to the BROWN wire anywhere that I can see.... If you put an Ohm meter on the ignition switch and find the PINK & BROWN terminals are still conected with the key in the OFF POSITION, you should put the diode in the DOTTED BROWN WIRE between your GREEN LIGHT & the fuse block.
And if that doesn't work as I claim, I'll cut my Tech Support Fee by 50%,
or you can pull my chain...!
My schematic looks a little different from what I can see of yours.
Mine is part # 500560 'FULL VEHICLE WIRING KIT 1960-66 CHEVY/GMC TRUCK'...
Danny