Originally Posted by andrewwynn

The 01 uses dual filament lamps for all four tail lights but only uses the low power filament for the upper lamp. I rewired mine to use the brake filament for the top lamps because I have some replacement light fixtures that have the filament wired for that ability.

As you noticed the plug on the harness is short a pin for this capability. I rewired my light fixture to supply power to the brake filament to top lamp and my car all four lamps light for brake. I tried to do the same for my wife's but the bulb is not wired to the high power brake only the low power run filament.

The lower lamp only uses the high power filament iirc, using pwm to change the brightness (I could have these backwards), this is why you often see different brightness between top and bottom lamp for the run lamp on e53.

It sounds like the designer for your tail light fixture did not take into account the weird wiring of the e53

Does the top lamp get even brighter when brake is applied or do they all become the same brightness?