No, my 95 has that socket but nothing is it in. I think they just reused the GM fusebox from the older Renix rigs and then added the newer relay box that contains the horn relay, combination flasher, rear defrost relay...etc.
However the vehicle in question is a 96. I know that by the different power distribution center. But I also know it isn't a 97+ because of the interior trim panels.
Do you see that little silver canister on that harness? And then the big black box hanging ALL of the way down resting on the floor?
Your combination flasher will be in there!
However the brake lights are not wired through the combination flasher. You may have a wiring problem (short somewhere) or whatnot. OR it might be a ground problem. Check ALL of the grounds for the rear lights!!