He means LEAVE THE BOOT OPEN but trigger the lock so the body computer THINKS IT IS CLOSED. Wait 16 mins (or there abouts) and check the COURTESY LIGHTS in the boot have gone out. If that tests good test for actual battery drain current and start to disconnect permanently connected items until the drain stops.
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL (X 2 - 1 Alpine White and 1 Glacier Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i