GMC C5000 Back Up Light Bulb Size

The GMC C5000 back up light requires a 1156 bulb size for the specific lighting component located at the rear of the vehicle. Available data covers the production range from 1979 through 1990 for the specified automotive application.

The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.

Bulb Size by Year Range

Years	Bulb Size
1979-1990	1156 (Medium Duty)

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. 12V, 21W.	1156

Other Locations

Additional Notes

The GMC C5000 (Medium Duty) uses the 1156 bulb for the back up light position across all listed model years from 1979 to 1990.
The 1156 is an ANSI-standardized bulb recognized across the United States and Canada, meaning it conforms to a defined set of electrical and physical parameters shared by all compliant manufacturers.
The 1156 operates on a 12-volt electrical system and draws 21 watts, which is consistent with standard incandescent automotive lighting of that era.
As a single-filament bulb, the 1156 serves only one lighting function per socket, distinguishing it from dual-filament types such as the 1157, which handle two functions from a single housing.
The 1156 uses a BA15s bayonet base, where "BA15" refers to the 15mm base width and "s" denotes a single contact pin, confirming its single-filament design at the base level.
The BA15s base is not interchangeable with the BA15d base found on dual-filament bulbs, as the pin configuration differs and a substitution would result in incorrect fitment or function.
All twelve model years in the specifications carry the Medium Duty designation, indicating these entries apply specifically to that vehicle classification within the C5000 lineup.