definitely the brake light switch (if they stay on all the time then its just that the switch is loose - so get a new switch is the best thing).
Under the glovebox for UK RHD cars you will see a two wire white plug that plugs into the rear of the brake light switch. Unplug this first & then pull out the brake light switch - it's not screwed or anything it just pulls out.
If the switch is working your brake lights will now be on (if igniton on of course).
So if they are not on then get a new switch.
Mine was £4.11 from allparts.
Refitting is direct reversal of above.
Colb
July 2009