Best way to isolate the drain is to pull the ground cable and hook it back up thru a multimeter with a DC amperage check function [even cheap $10 meters have it]. This way you can see how much curent is being drained, then disconnect bits or pull fuses till the flow stops. Start with the radio/clock fuse as that's an automatic trickle, what's left [if anything] shoud be your mystery drain. Have someone watch the meter while you pull fuses.