Originally Posted by wpoll

It most likely is the after-market wiring...

The BFD feature is provided via PWM to the top lamp. The lamp is a 12v 21W single filament lamp and as a tail lamp it has a PWM pulse duty-cycle of about 20-30% - which makes the 21W lamp glow dimly like 5W or 10W tail lamp. When the BDF lighting is called for, the PWM duty-cycle changes up to about 90-100%, so the lamp glows like a 21W (brighter!).

LEDs cannot easily vary their brightness, although oddly enough the best way to dim LEDs is with PWM, but not quite the same system that BMW use on the OEM tails. So the after-market LED tails glow the same regardless of the PWM duty-cycle. The manufacture then has to wire them such that they are either on or off - possibly in groups to provide both tail and brake lighting behind the same lens. And the capacitors that are used to removed the "flicker" (which act as averaging devices) also screw up any chance of using BMW's PWM on after-market tails.

I'm guessing this is why there are so many after-market LED tail options for pre-facelift cars (which don't use PWM) and hardly any for facelift cars - it's just too hard for the after-market manufacturer to deal with the PWM.