Brake lights

The first suspect is the ground wires on the sockets. They plug in on the side if you have the older-style round brass base bulbs and often corrode. If only one side is acting up, turn on the turn signal on that side. Typically the bulbs will light up but not flash. Remove the bulbs from the other side and the lit bulbs will go out. That's because they're getting a ground, but it's a poor ground, from the good side.

If you have the newer-style 3157 bulbs with the flat plastic bases, pull a socket out so you can measure the voltages on the wires on the base with the bulb still in the socket. Turn on just one thing, ... Tail lights, turn signal, or have a helper press the brake pedal. There's four wires on the bulb's base. You should find 12 volts on just one of them. If you find 12 volts on one and around 6 - 8 volts on the others, you have a bad ground. Two terminals in the socket will have the same voltage. It will be lower than 12 volts but higher than the last terminal. Follow that wire to where it bolts to the body. Look for a corroded terminal or break in the wire.