As Jasper says it's probably the rear brake light switch. They're such a terrible design as they open directly into the path of travel, so all the dirt and water get in and it either rots away or gets clogged up and doesn't return.
Get it off and give it a liberal spray with some WD40 then a good seeing to with an old toothbrush. You might be lucky and be able to resurrect it, but if not it might be worth replacing the switch with an inline pressure switch, as this will last a lot longer and be trouble free. You can sometimes pick them up cheaper than a new genuine switch:
http://cgi.ebay.com/ebaymotors/ws/eB...m=140326699343