attach a current (amp) meter to the battery's negative terminal and begin monitoring current flow (engine off, all doors closed, etc) It shouldn't be any higher than 30-40 milliamps or so.
If much higher - starting pulling fusible links until it drops to zero. Report back on which link is causing the issue. If the amps are wired directly to the battery (bypassing the fusible links) -- be sure to disconnect that circuit too as part of this test.