There should be 2 wires used to power the coil, a "start wire" and a "run wire"
The start wire gives 12volts power to the coil only when engine is cranking.
When key is in the "run" position the run wire gives the coil 8volts to keep it from overheating.
I reads like you have a problem with the "run wire", this wire is either a "resistor wire" or a wire connected to a "ballast resistor" that is connected to the coil.
Symptom of a failed "run wire" is: Engine fires when cranking but dies when key returns to "run" position.
If you have two wires on the "+" side of the coil one will run to the Starter Solenoid, it will either connect to it's own "i" post or connect to the Starter Motor post on the solenoid, this gives the coil 12volts when engine is cranking, solenoid activated.
From your description the other wire is the one that is not working.
Sometimes the resistor part fails, it does get hot reducing the 14.5volts(alternator) down to 8volts, you can run the coil temporarily from another "key on" 12v source, but it can overheat the coil and cause it to fail prematurely.
You can replace the "run wire" with a resistor wire or a ballast resistor, from a key on 12v source, if you can't trace the wire problem.
And check your voltage at the battery, often the cause of the "run wire" failure, and Coil failure, is because system voltage is running to high, 14.5volts at 2,000 RPM is about as high as it should be.
13.8volts at idle after battery has been recharged after starting, a minute or so.
EDIT:
Couldn't find an '87 ignition diagram but did find an '89, the '89 doesn't show a separate "run wire", the key switch has the "start and run" both.
So test the "+" side of the coil for 12volts with key on/engine off, if it has 12volts then the above is not the issue.
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