First of all the abs system, when there are faults displays a light on the dashboard where the speed and rev is..
on the obc = on board computer, the one on the center console, the brake warning light problem is caused by some reasons, first check to see all the lights are working, and even if they are make sure the contacts are clean, using a good contact cleaner helps. then there is the brake light swicth on top of your brake pedal, there was a recal on this. the swcth works, but because o defect on production, the swich has a higher internal resistance, therefore trigging the message.
the way this sistem works is by monitoring that there is a closed look on the light circuit, if there is a light that if burn out, then the loop is broken and so it triggers the computer. the way it monitors the loop is by resistance on the wires, if there is too much resistance build up it will trigger the computer.
so with time, moisture creates rust wich in turn creates electrical resistance on cundoctors. then the brake swicht when working also creates big resistance and it makes the obc mad.
Rule to fix this. first check the light bulbs, the light holders and use contact cleaner to clean all metal contacts on the tailight. also check if the light number that is on that hole macthes the factory specification. all lights have a universal model number this gurantees that you are using the specified light for that job, light bulb specification should be on the onwers manual..
if after doing this the message comes back, then replace the swicth, get it at dealer or ebay...
and the worst case scenario that should only be done if the above does not work, replace the obc light monitoring box. this light monitoring box sits under the dashboard on the drivers side, us spec. its a little blck box with some 2 big wire clips and several wires on them. but then again this is very unlikely.
what i did find out on my car was that even after doing all the above the message comes back because of a simple problem, the tailights itself, they use a cheap metal as a conductor inside of them, and with time the contact between the light holder and the metal gets worn out so contact is deficient., so before replacing the computer, i would go and get the clear lens tailight to fix this. make sure you get good quality ones, as the cheap ones are not made to work on light monitoring systems and you have to adapt it. use bavauto.com they have ones that work like a charm..
hope this helps.