The electronic brake light switch only operates as from terminal R ON.
It has its own voltage supply.
If the brake light switch is OK, it assumes a high-resistance setting when the ignition is switched off, i.e. it sends a signal as if it were operated. If this is not the case, this indicates a short to ground and the messages "brake light defective" as well as "rear right lamp defective" and "rear left lamp defective" are output or indicated in the pictogram in the instrument cluster.
The light switching centre recognises the "operated" status if there is a break in the line. If is recognises the switch interrupted for longer than 10 s as operated, the light switching centre then also makes use of the vehicle acceleration signal. If a conflict is then detected, the above-specified error messages are output.