[joeval]

Since your top brake light works, I would check the switch at your brake pedal. It's likely, there is more that one wire meaning that one will control the top and the other the sides. If there is current coming out (volt meter or test light) when the brake is pressed, then check the connections at the light to make sure they are not corroded or dirty. Sometimes, just cleaning them and making sure they are making good ground will make a difference. If it doesn't work, check for current at the light when the brake is pressed. If there is none, check the wires behind the connections for damage. If none of this works, then it gets trickier.