The GMC C1500 brake light utilizes unique bulb sizes 3157, 3057, 1157, and 2057 depending on the specific version of the vehicle. Available data for the components covers the range from 1979 to 1999.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The GMC C1500 brake light specifications reference four distinct bulb types across model years 1979-1999: the 1157, 2057, 3057, and 3157. All four are dual-filament bulbs operating on a 12V system, designed to serve both the brake (high-intensity) and running light (low-intensity) functions from a single housing.
The 3057 and 3157 are T25 wedge-base bulbs sharing the same 12V, 27W/7W output rating. Both conform to ANSI standards for the US and Canadian markets. Despite their near-identical electrical characteristics, they are distinct part numbers and are not universally interchangeable without confirming socket compatibility.
The 1157 uses a BAY15d bayonet base and operates at 12V with a 21W/5W dual-filament rating. It is among the most widely used automotive bulb types in North American vehicles and appears in the GMC C1500 (1979-1982 and 1993-1997) across multiple non-consecutive model year groups.
The 2057 also uses a BAY15d bayonet base, making it physically compatible with 1157 sockets. However, its wattage rating differs at 26.9W/8W, which places it between the 1157 and the T25 wedge types in output. It is described as a brighter variant of the 1157.
Because the 1157 and 2057 share the BAY15d base, physical fitment in the socket does not confirm electrical equivalence. The wattage difference between the two types means substitution may affect brightness and could place different load demands on the vehicle's electrical circuit.
The T25 wedge bulbs (3057 and 3157) and the BAY15d bayonet bulbs (1157 and 2057) use fundamentally different base types and are not interchangeable with each other, regardless of voltage or wattage similarities.
The Center High Mount Stop Light (CHMSL) on the GMC C1500 uses a separate, lower-wattage single-filament bulb. The 921 is a T15 wedge type rated at 12V and 16W, while the 912 is a variant of the 921 in the same T15 form factor but rated at a lower 12W output.
The 912 appears exclusively in the 1998 CHMSL specification, while the 921 covers 1993-1997 and 1999. Both the 912 and 921 share the T15 wedge base, meaning the socket form factor is the same, though their differing wattage ratings distinguish them as separate specifications.
All bulb types listed across both the brake light and CHMSL specifications conform to ANSI standards applicable to the US and Canadian markets, indicating standardized dimensional and electrical tolerances across manufacturers producing these bulb numbers.
The brake light bulb specification for the GMC C1500 changed multiple times across the 1979-1999 production span, with no single bulb type covering the entire range. This means the correct bulb type is year-specific and cannot be assumed consistent across the model's full production history.