Turn everything off, close doors and boot, leave one window down. Connect amp meter between battery lead and battery terminal. You should be able to read the draining current. Disconnect amp meter and unplug light switch. Close up car (apart from window) and connect and recheck ampmeter. Keep doing this to other systems (alternator, ign switch, boot light, glovebox light etc) until drain current stops. Fix that system.. (I keep saying leave a window down in case central locking cuts in - would not be the first time..........)
Good - Fast - Cheap: Pick any two.
E23 735i; E32 735iL (X 2 - 1 Alpine White and 1 Glacier Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E30 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i