I just ordered some from the dealer. I was missing the whole thing. I got two "plungers" as they call them. Two grommets, the metal thing that holds them and the plastic cover for a grand total of 50 bucks. I already tried the junk yards but they only sell the whole 3rd brake light and not individual parts. The "plungers" were the "cheap" parts at like 3-4 bucks. They normally aren't in stock at the dealers so expect waiting a few days.
'03 TJ Sport
/ D30/Ford9 / 35x12.5x15 KM2 / D window black steel 15x8 wheels / 3" custom long arm lift / SYE & 1350 DS / JKS BL / 1" MML / Smittybilt's front bumper, Tube fenders, Rocker guards, and rear bumper / High Rock Tire Carrier