Figure out the amp draw on the battery. Pull fuses till it goes away. Your problem will be on the circuit that has the draw. Aftermarket equipment is always the first suspect.
^^^ This
A parasitic current draw may be small enough that an inexpensive DVM won't adequately measure the current.
You may need to get yourself a DVM that measures both amps and milliamps (mA). Most of the inexpensive Fluke meters will do this, and old Flukes are relatively cheap on eBay (e.g., the original Fluke 75 or 77, or their later "Series II" or "Series III" counterparts). I say "relatively"; looking just now, it seems any decent Fluke is north of $80. Well, you get the gist: A better meter may be required to measure "mA".
Some of the DVMs that measure milliamps have an internal fuse of their own, for protection. If the internal fuse blows / is blown, the meter won't measure current until it's replaced.
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