Originally Posted by kevkon
x2 on the ground and check to see if there is some trailer wiring that was added. Another possibility is the hazzard switch connection(s).
Agreed on checking all that, especially if someone botched in trailer light wiring.
You have a short issue most likely as the brake lights and the signals are on different circuits, so something is messed up. A bad ground would typically make one side dim or do other funky things, but if you are getting the brake lamps to come on when you run a signal you have a wiring issue and you will need to trace the wires and see what is happening.