Well, based on 2000 wiring diagrams, which I think are the same as '99. the good news is that there is nothing in the least bit complicated about this system. The brake lights go back directly from the brake light switch to the bulbs to ground. And the ground is common with the tail light ground. So if the tail lights work, it is not the ground. Fuse is #23 in the PDC (box under hood). No relays. White and tan wire from switch all the way back to the lights. Most likely scenario is a broken wire somewhere between the brake switch and the point where the wires split to go the the individual right and left lights. Hard to see both light sockets having a contact issue, but on trailers, I've seen it happen. I'd double check for voltage in the socket with someone holding the brake pedal down (or jump the switch) before I started tracing the wire.