Hi Tom and welcome to JustAnswer,
Common areas to check are the wiring harness that runs along the left side of the trunk hinge. Peel back the black wire loom and inspect for broken wires. Pull the loom back as far as where the harness splits of into two. That's where the wires usually break. If you found some broken wires, repair that first. If not, then you need to inpect the brake bulbs. Make sure they are Sylvania brand because the Lamp failure ECU is very sensitive to different wattage bulbs. I have replace the 3rd brake light bulb to resolve the issue. You might want to do that if you haven't already. Lastly, if the bulbs are all good and the right type and you;ve replaced the 3rd bulb, then you have a shorted out lamp failure ECU. This ECU is located left of the trunk, behind the carpeted panel. This ECU is not cheap and they cost around $200.