electrical gremlins?
 

Turned the engine on, stepped on the brake pedal - a warning triangle on the instrument cluster comes on, below the message: taillamp failure
Checked: both tail lights are on, both turn signals working, both brakelights working, high brake light on

How do I erase the message?

This warning is misleading - it should say "exterior bulb failure". Both front headlights have small lights that when burned out (or replaced with LEDs) will throw this taillamp failure message.

Also, check that the dual-filament bulbs on both sides are fully working. If 1 low power filament is working but the high power one isn't, the system will recognize that the bulb has failed.

do all bulbs have dual filaments? To me all bulbs seem to work fine. I see no different intensities (raining hard)

the message says Taillamp failure
It comes on only after I activate the brake pedal, no message when just starting the engine.
Do I assume correctly that it pertains only to the bulbs in the brake light?
How difficult is it to change the bulbs? I checked some diagrams but nothing shows how to remove the bulbs, only a code # for the bulbs (presumably available at the dealer)
PS: thanks for the electrical diagram but for a construction guy taunting (to say the least) ** I will file it nevertheless**

fixed
It was not the brake light bulb but the lower left tail light bulb. This bulb has a dual filement - one of it was burned out and caused the warning message
For the ones who has never exchanged the bulb: the small "trapdoor" is fastened with a velco strip; it also has a hinge on the side facing the outside of the car. If you pry on the hingeside you might damage/brake off the little door. So, pry on the side facing the trunk. The bulb holder is also quite tight; tight to get at it and tight in the holder itself. You loosen the holder by twisting it, then pull hard.
The bulb was C$ 7 for a Philips long life (2bulb package)
The message did not come on any longer after the bulb was exchanged