Has the wiring be tampered with by a PO or does it all look to be original.
Sounds like the Alternator charge is getting a direct route to the coil some how.
Disconnect the alternator, then start it and see if it stops when you turn it off.
Then its a matter of tracking down where the power is getting the connection to the coil, Time to get the DMM out, also search internet for an electric diagram ( plenty of D1 manuals out there for free)
Main fuse box may be suspect as a good place to look?
Even though you said you have checked the ignition switch I wouldn't rule it out yet
No relays on a D1????
I dont know the D1
But think the D1 was a pretty basic starting/charging/running circuit.
J