All it takes is one wire accidently scrunched down or rubbing against the body metal.

Service manual is best.
'69 B Body I'm not that familiar with.
Schematically, it will be almost the same as any late 60s A or B body.
There was definately some change in the connections, and I think the tach wire may have been run through a grommet in the firewall?
Could do a resistance check of the coil itself. Primary connections should be open to ground.

Not sure if you looked through the link above.
It should be helpful.

But for a general scheme of the main circuit (power feeds and everything connected to them) this will be what you want to make up or alter the details for your car.
First one doesn't include the start circuits. If there's no probelm in them, its easier to leave them off the schematic.
Key in RUN, both Ignition 1 (Run) and Accessory feeds get power from the switch. Accessory includes power to flashers, windshield wipers, and two or three circuits with fuses.