engine died today....

The factory wire that provided power to the coil "+" terminal, pink I think, the resistance wire, has power when the key is turned to IGNITION-ON, and no power at any other time. Put a voltmeter or test light on it and verify that it is so. It should always go on with the key and off with the key, no exceptions.

If somehow you have power on the relay coil pin 86 when the key is OFF, then it's wired wrong. Or, the relay has failed and is sticking on, mechanically, inside. Swapping relays should identify that.

There are some incredibly craptacular relays out there. Definitely an item to not buy by "lowest price". Buy actual Bosch brand (which probably means not from Amazon). For the last couple years I buy mission-critical generic stuff from Summit. Here's one that's probably decent: https://www.summitracing.com/parts/bch-0332019150

Thanks for chiming in, Tom.  Just checked the voltage to the coil and 

it's 12v when the key is turned to ignition--on; off when ignition is off.

And, relay clicked when the key turned to ignition--on.

No harm in having a second one there also.

The distributor grounds through the housing through the block through the block ground wire.

The ignition should only get a full 12 volts when CRANKING / Starting.

Original factory wiring & the Solenoid attachments accomplish this.

But once the ignition key is released and returns to the "Run" position

the coil / ignition is then being fed through the factory resistance wire and less than a full 12 volts.

If you're seeing a full 12 volts in the run position something is different, not stock.

BUT the MSD ready-to-run Distributor might want it that way.

Gotta ask, the Battery Hold down clamp is in place and tight?
If the battery tilts and the battery post touch anything, bad things happen.

Yes, battery is effectively installed and no issues with that, 

AFAICS.

was somewhat easily accessible from underneath the drivers side dash.

That is when a person's young and limber and can easily contort them self in there LOL

Familiar with all this as long ago I did an incognito in-dash "Shift Light" by commandeering

the optional tacked-on simple stand alone E-Brake Warning Light Harness and Bulb Holder,

disconnecting it from the E-Brake mechanism Ground Switch, & attaching it to an MSD RPM Switch.

An there it was, a hidden, stock, in-dash RPM Activated Shift Light... and a UniLite fired MSD 6 Box.

Car also had the optional Headlights on Warning Buzzer, which was still functional !

It all worked fine for awhile after initial install, then one evening acted up similar to your event.

That is when & how it had to be dug into and figured out, and found the "back feed" situation.

Just so everyone knows, not just blowin' smoke out of my... and makin' things up...  Peace