On the XR, BMW uses the same lamp for the brake and running lights. They modulate the signal being sent to this lamp with a 10% duty cycle pulse when the brake is off, so the lamp lights up at a lower, running light intensity. When the brake goes on this duty cycle is increased to 100% and the light increases to maximum brightness.
The Denali T3 has no intelligence in it, and so cannot distinguish between these two modes, which is why it doesn't work. Many bikes use this method, so the product is of limited usefulness unless your bike uses the traditional two filament tail/brake bulb with separate running light and brake light wires.
In order to make supplementary brake lights work on our bikes, you need a lights with a smart controller such as P3 Lights from Skenelights. Its computer differentiates between the two signals sent on the brake wire and lights up its supplementary LEDs accordingly. A Canbus device like the Cansmart can also be used as mentioned above, but this adds significant cost on top of the T3 units.
(Full disclosure - I designed P3 Lights and am affiliated with Skenelights.)
