If you look between the right brake lever on the bars and the mount, you'll see a tiny switch in there that contacts the handle when you release the handle to break the circuit that light's the brake light. If the handles were removed and reinstalled, it's a good chance that the brake light switch was broken off in the process if it was not done right. I've also seen them where they needed a tiny touch of WD40 when they were sticking too.

This link will give you some understanding of the disassembly/replacement of the switch housings.

http://www.box.net/shared/rc0q9bjltd

This is what you have to do when you remove the brake lever housing so you don't damage the switch.

http://i149.photobucket.com/albums/s45/bills985/switchhousingspacer.jpg