One other possibility, although remote, is that the starter is leaking current.

To test this, you would need to remove the positive cables from the starter, however, the cable from the battery needs to remain in contact with any other cables that are now disconnected from the positive terminal on the starter.

If the reading now drops to near zero, the starter is the culprit.

If neither the starter, nor the alternator, nor any of the circuits in the fuse panel are the culprit, I would need the electrical schematic for the SD to attempt to go further. I have a newer schematic for the SDL, but, have not found one for the SD.