Is it the brake lights only? If so, it's probably not the ground connections. Could be a faulty brake light switch. Look at a wiring diagram and trace where the wire goes to at the switch. Pull it off and get a length of wire and connect one end to + at battery and the other to the end of the wire you pulled off. If the wiring to that point is ok, then the lights should go on, in which case, it is either the wiring back through to the + source or the switch itself, or the contacts at the switch. You can also just simulate the operation of the switch by just pulling both terminals off the switch and shorting them together with a short piece of wire. If that works, then it's definitely the switch that's bad. So give that a shot and see if that does it.