The "upside down Xmas tree" symbol is a ground symbol. The 1034/1157 bulbs ground through their housing and then through the socket and metal taillight housing. Plastic housings have an extra ground strap connection. Just test the brake light switch in place. It's just a 12V power wire in, and a load wire out. One terminal should have power all the time, and the other will have power when you push the brake pedal down and the switch closes. Those switches can go bad, but, it's not real common. They're a pretty simple part.