This is as much as I can figure so far.
The biggest question is how does the alternator output connect to the ignition and battery?
View attachment 1715268280
Switches
1. Tach. But maybe also something else?
2. Start. Apparently powers starter relay?? In which case, S2 yellow has to ground somewhere.
3. Run. Power to ignition coil and MSD 5900
4. Gage Illumination.