[SikhBMW]

I noticed the inner tail lights do not light up when you press on the brake. They are aftermarket however i do not remember if they do on factory tail lights. I also changed the bulbs to LED. Reverse (top bulb which is on in the image)
however the area annotated doesn't light as the outer ones do.

[ezaircon4jc]

Originally Posted by SikhBMW

I noticed the inner tail lights do not light up when you press on the brake. They are aftermarket however i do not remember if they do on factory tail lights. I also changed the bulbs to LED. Reverse (top bulb which is on in the image)
however the area annotated doesn't light as the outer ones do.

The inner (trunk) lights only light when brake force (I don't remember the official name) is coded.

[philipm785]

I think they light up under hard braking (a certain amount of braking force is applied). Probably will be difficult to observe.

The brake force display can be coded to flash under heavy braking (I think this is the default in the EU?), but here in the US it's coded by default to "area enlargement" which means that the inner lights come on as well as the outer ones.

[Qsilver7]

Originally Posted by philipm785

I think they light up under hard braking (a certain amount of braking force is applied). Probably will be difficult to observe.

Ditto...if you can find a large reflective surface (like a building that has a large glass front or lots of glass windows/doors and enough parking space for you to accelerate away from the building then emergency brake...then between dusk-to-dawn you can accelerate rapidly then brake hard...while looking in your rear view mirror you can then see the "BRAKE FORCE DISPLAY" lighting illuminate that part of the rear tail lights. If you're in Europe, then you can see the "Dynamic Brake Lights" flash if you can meet the speed & braking force requirements.

See the info below from the TIS website that describes the two features and how they operate:

Depending on the series and national-market version either Brake Force Displays or dynamic brake lights are used.

Brake Force Display (USA Version)

• With simple braking, nothing has changed on the system used to date: the brake lights light up "normally". However, if the braking is harder and the Dynamic Stability Control (DSC) determines a deceleration of more than 6 m/s2 - this corresponds to approximately to braking where a briefcase would slide from the front passenger seat - another field lights up in the left-hand and right-hand brake lamp,. or additional LEDs are activated in the brake lamps. Following road users this register the two-stage lighting of the brake lamps or a significantly stronger brake light than normal within a few tenths of a second every time there is strong braking or in the event of emergency braking.

Dynamic brake lights, flashing (European version)

• The function with the flashing dynamic brake lamp is triggered as of a driving speed of over 50 km/h and it takes place in a number of phases:
  
  • Normal braking
    
    • With simple braking, nothing has changed on the system used to date: the brake lights light up "normally".
  • Emergency braking
    
    • Under braking deceleration of more than 6 m/s2, the footwell module (FRM) or the front electronic module (FEM) or the Body Domain Controller (BDC) switches from brake light "constantly on" to "flashing".