'84 300SD Electrical Problems

I have an W126 300SD that has some strange electrical problems. Although I've had the alternator and battery replaced a couple times, the battery rarely stays charged. If I drive for a long period of time pretty much
everything electrical in the car stops working (radio, windows, turn signal, etc.) by the end of the trip. Right now the car is driven maybe twice a month, but the battery often needs to be recharged to start. Any suggestions? Would a Mercedes dealer have a better chance of figuring this out than a regular Mercedes mechanic? Also, I'm interested in selling the car once I get it working properly. Does anyone know of a good place to sell a car on-line?
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My first thought was that the engine's (therefore alternator) ground connection to the car's body is suspect. That would explain the battery rundown during driving BUT, if were so, then the starter wouldn't work. So much for that idea.
Either the alternator isn't charging or there's a serious current drain.
The alternator has been replaced a couple of times - with a Bosch alternator? Or brand X junk?
You can sequentially remove each fuse and put an ohm meter or 12v test light on its circuit to determine if there's a drain when the engine is shut off. There should be no current flowing other than in the clock's circuit.
If the car was ever wrecked the crash site should be checked for a pinched or bare wire that may be shorting and draining the battery.
Another suspect is the power antenna; its power supply line can be unplugged as a test.
A dealer will be only too happy to take your money, especially for investigating something so open ended. This isn't rocket science but a process of elimination.
If you can't find the problem, take it to an auto electric shop.
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