GMC G2500 Back Up Light Bulb Size

The GMC G2500 back up light uses 1156 and P21W bulb sizes according to the provided technical specifications for the vehicle. Available information covers the range from 1979 through 1995 for the specified lighting components.

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

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Additional Notes

The GMC G2500 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 in both the United States and Canada, meaning it conforms to a defined set of electrical and physical parameters shared across manufacturers.
It is a single-filament bulb, which is typical for back up lights since the function requires only one lighting state rather than the dual-intensity operation found in combination stop/tail applications.
The 1156 uses a BA15s bayonet base, where "BA15s" denotes a bayonet mount with a 15mm collar and a single contact pin, distinguishing it from the dual-pin BA15d base used on some other automotive bulbs.
The bulb operates at 12 volts, consistent with the standard electrical system voltage of passenger and light commercial vehicles of this era.
Its rated wattage is 21W, which places it in the mid-range of single-filament automotive bulbs and reflects the power draw expected from the GMC G2500's back up light circuit.
The BA15s single-pin base is not interchangeable with BA15d dual-pin bases, so substitution with physically similar but differently pinned bulbs would result in an incompatible fit.