Perhaps this is the reason your car was on the market.

Sometimes you need to go to a dealer to find a problem before you "parts" yourself to death.

While expensive, it might be far cheaper than losing other components while you try ti figure things out.

I do not know for sure, but I think that a leaky diode could run your battery down.

I had a 1972 Honda that would go dead if not used for a few weeks, I found that the rectifier leaked 450 ma - that reading was high, but at the top of spec - I had to disconnect it if I planned to let the bike sit for several weeks. BTW - Honda had wired the rectifier so that it was convenient to unplug it - perhaps in recognition of a poor system?
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '95 855, '95 854, the first three are mine, heh, heh, 485,000 miles put on 9 bricks