1. hook up an ammeter at the battery (fully charged, key off)
2. note current
3. pull 1 fuse
4. note current, compare with current in step 2.
4. a. no change in current? replace fuse and go back to step 3.
4. b. current reading change to zero? congratulations, you found the problem circuit. Troubleshoot the circuit.
4. c. current changed, but not zero? you may have more than one faulty circuit, or a non-trivial problem. Post results, maybe the forum can help more. good luck.
5. gone through all fuses with no change observed? Disconnect alternator, note current.
6. Still no change in current noted? Find electrical schematic to your vehicle and post in this thread, along with readings from the above tests.

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Remember, Safety Third!
'99 E300 Turbodiesel, '82 300TD, 1996 12V Cummins Turbo, '94 Neoplan - Detroit 6V92TA