I have always heard that removing the battery while the car is running - on an alternator system (not the really old generator cars) - will destroy the voltage regulator. Not sure if its a transient that gets it or an overvoltage condition or what - but I would discontinue that test where you pull off a battery cable while the car is running.

Glow plug relay is on the driver's side fender under the hood, a bit towards the front of the car from the middle of the wheel. It has a cover over the socket and plug connectors. If you are getting a battery drain from a glow plug relay problem, you could disconnect the glow plugs connector after startup and make your drive. This would get old after a while, but might serve as a temporary workaround.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles