Replacing the third brake light (CHMSL) on a 2000 GMC Sierra is a straightforward task, but it is also a critical maintenance item. Over time, the factory gasket on this light degrades due to UV exposure and temperature changes. For owners of this generation Sierra, a failed third brake light gasket is one of the most common causes of water leaking into the cab, often running down the back wall of the truck and soaking the carpet or even damaging the audio system. Here is how to replace the light assembly and, more importantly, ensure your cab stays dry.
Step-by-Step Replacement Process
To access the third brake light, you need to remove the two exterior screws located on the top of the cab, directly above the rear window. Use a Torx driver (usually T15 or T20) to remove these screws. Once the screws are out, the housing may feel stuck due to the old, deteriorated foam gasket acting like glue. Do not use a screwdriver to pry it off, as this will damage the paint. Instead, gently rock the housing side-to-side or use a plastic trim tool to break the seal. Lift the assembly out and disconnect the wiring harness by pressing the tab.
Before installing the new light, clean the area thoroughly. The roof panel where the light seats is likely dirty and may even show signs of previous water tracks. Use rubbing alcohol or a wax remover to clean this surface. Install the new light by connecting the wiring harness first. Seat the new gasket flush against the cab, reinsert the screws, and tighten them firmly—but do not overtighten, as this can crack the new lens or compress the gasket unevenly.
How Headlights Relate to This Job
While you are focused on the rear of the truck, this is an excellent time to check the integrity of the headlight assemblies at the front. Moisture intrusion is a full-truck issue for the 2000 GMC Sierra. Just as a bad seal on the third brake light leaks water into the cab, a cracked seal on the headlight allows humidity and dirt to enter the front housing . This leads to foggy lenses, reduced light output, and premature bulb failure. When inspecting your truck’s lighting, check the 2000 GMC Sierra headlight gaskets for cracking. If the headlight shows signs of internal condensation, you need to reseal it with butyl rubber before it causes electrical issues at the front connector, mirroring the preventative work you just did at the rear.
Preventing Future Cab Leaks: The Gasket Upgrade
The factory foam gasket on the 2000 Sierra is the root of the problem. To ensure you never deal with a wet back wall again, do not rely on the thin foam that comes with many replacement lights. Instead, purchase a sheet of closed-cell foam rubber or silicone rubber (1/4 inch thick). Trace the outline of the new light housing onto this sheet and cut a custom gasket. Alternatively, apply a bead of high-quality outdoor RTV silicone (like GE Silicone II) to the mating surface. Allow the silicone to skin over for a few minutes before tightening the light down. This creates a watertight barrier that will outlast the truck.
Finally, perform a water test. After installation, take a garden hose and run water over the roof and the rear cab window. Sit inside the cab and look for drips. By upgrading the gasket material and aligning the housing properly, you will permanently solve the cab leak issue for your 2000 GMC Sierra.
