I've seen batteries that were flatter than the paint on the garage floor when they were brandy-new. What makes me think 'battery' is the fact that it starts and runs when jumped, and your measurement of 14 volts when it's running. The alternator is what raised the system voltage, and nothing on the car draws as much current as the starter. That's why you could have lights and such working, but no start.

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1989 300 SEL that mostly works, but needs TLC