1999 Ford Fiesta brake lights coming on when accelerating?

A friend following me today said that my brake lights kept coming on even though I was accelerating. Not had a chance to check this for myself yet, any ideas? I also discovered black hair hanging out of my exhaust but after initially thinking someone had stuffed an old wig up there, have since discovered it is probably the "back box"? Is that the same as a silencer? I've had more work/replacement exhausts on this car than I've had on all my previous cars put together. Could there be a cause for this or do they just don't make them like they used to?
many thanks