I don't think there is a relay for the tail-lights. Your dash lights are working?? Check fuse 15 in the fuse box. That is, I believe, the tail light fuse. Would not affect the brake lights though - The FACT the center brakelight is working AND the signals are working leads me to suspect a damaged harness or connector in the trunk - something that has been put into or removed from the trunk has hit either the connector or the harness.
OR it could be the bulb failure warning unit under the dash part number PN 1362370 - behind the driver's side knee bolster near the stearing column - it should be red. I think there are 2 yellow wires coming out going to the brake lights - both should have power with the brakes on. The tail light assemblies on those cars are also NOTORIOUS for having problems. check for power at the connector to the tail-light assemblies. I think the power for one side runs through the other side
- can't say which one is the "common" side - but if it fails the other sides doesn't work either . It will be the one that the main harness connects to - I THINK it is the left. If it was here I'd likely find the problem in less than half an hour - but troubleshooting long distance on a car I have not seen or worked on in over 15 years is problematic - particularly without the manual/ wiring diagram.