Slow Battery Drain

The suggestions above are good but one thing left out.

charge battery, yes
load test it yes
check for draw with test lamp and them ammeter yes
If you can't find it, LEAVE THE BATTERY with one post disconnected a couple of days and see if it goes down
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IF IT is a draw. Put a series device in the ground that will show the draw, whether a meter or test lamp

Start pulling fuses. Be sure to include the dome lamp as you'll have the door open. Do not forget the trunk or underhood lamp or the GLOVE BOX lamp
Cigarette lighters can "leak" when dirty/ corroded/ rusty.
If nothing shows up this way, disconnect the alternator and then re-check (Leaky diodes or leaky radio cap)