Just put an ammeter in the main line to the battery when the key is off and all of the lights are out and see what the standby current draw is...it should be near zero. If it is then I would look to the charging circuit.

If there is some draw like 100ma or more drawing at standby then start pulling fuses out until it drops off and then you will know what circuit it is that is draining it. From that point you can pinpoint what device on that circuit is the culprit.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz