This test: measuring the voltage between the batt cable and the batt terminal won't work on a modern car. Virtually all cars have some form of retained memory in control units that draw very little current but show full battery potential.
The proper test is to hook your multi-meter between the battery and the cable in the Amps setting. Do this with everything shut-off and with the ten amp scale (first - till you see the size of the load). You still could blow a tester fuse if a blower comes on.
Each car is rated on its maximum parasitic draw, but I would say your current flow should be less than 50ma. A trunk light bulb on will be it the neighborhood of 200ma.
If you have current above these levels disconnect circuits to narrow the search. Use the volt meter and identify the fuses that are powered when the key is off. Pull these fuses one at a time till you can see the current draw fall (on the ampmeter). There are probably some circuits that don't go through the fuse box so a wiring diagram is the next step. I would try unplugging the antenna, the alternator, the trunk light bulb and the fuses one at a time.
Of course all of this should take place after making sure the alternator is charging. If all you have is a volt meter a fairly good test would be to measure battery voltage at idle and 2000rpm. The voltage should be above 13 and below 14.8. If you turn on the headlights the load wil drop the voltage at idle but it still should be greater than the battery voltage with the ignition off.
Owner 24 bay BSC
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26 years MB technician