It sounds like the light control module (LCM) is defective. Never had to fix mine but read about the problems. Try: Remove; terminal and earth clean and refit B4 you replace it. Also check out the wires connectors and earth points in the boot. You could find the problem or fix it with out seeing anything wrong.
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL (X 2 - 1 White and 1 Light Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i