When I hit the brakes, the tailight on the driver side won't light up. What's the problem?

Check the bulb first, be aware that the newer bulbs fit into a slot that has very thin contacts in them, and the ones on the bulb are the same size, if you try a new bulb, make sure the lights are all off, if it doesn't work, and very gently pry that wire out from the plastic that houses it,, at times they get smashed in there and won't make contact