GMC G1500 Back Up Light Bulb Size

The GMC G1500 back up light utilizes 1156 and P21W bulb sizes for the rear illumination system. Available information spans a time range from 1979 through 1995.

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
1979-1995	1156

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 G1500 back up light bulb specification calls for the 1156 bulb across all listed model years, spanning 1979-1995.
The 1156 is an ANSI-standardized bulb type 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.
A single-filament design defines the 1156, meaning it serves one lighting function only. In the context of the GMC G1500, that function is the back up light.
The 1156 uses a BA15s bayonet base, where "BA15s" denotes a single-contact bayonet fitting with a 15mm base width. The single-contact design is directly tied to the single-filament construction.
The BA15s base is distinct from the BA15d base, which is a dual-contact bayonet fitting used on two-filament bulbs. These two base types are not interchangeable despite sharing the same 15mm base width.