GMC S15 Back Up Light Bulb Size

The GMC S15 back up light utilizes 1156 and P21W bulb sizes for the rear illumination system. Available data for the specified vehicle covers the period from 1982 to 1990.

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
1982-1990	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 S15 uses the 1156 bulb for the back up light across all listed model years, spanning 1982-1990.
The 1156 is an ANSI-standardized bulb type, meaning its specifications are governed by a recognized North American standard applicable in both the United States and Canada.
The 1156 operates on a 12-volt electrical system, which is consistent with standard automotive DC voltage.
At 21 watts, the 1156 produces a relatively high output for an incandescent single-filament bulb, which is typical for back up light applications requiring adequate rear illumination.
The 1156 uses a BA15s bayonet base, where "BA15s" denotes a single-contact bayonet fitting with a 15mm base width. The single-contact design corresponds directly to the bulb's single-filament construction.
Because the 1156 is a single-filament bulb, it serves one dedicated lighting function per socket, as opposed to dual-filament types that handle two functions such as braking and running lights simultaneously.
The BA15s bayonet base requires a twist-and-lock installation method rather than a screw-in fitting, which is a relevant distinction when sourcing a replacement for the GMC S15 back up light socket.
The 1156 designation is shared across a wide range of vehicle makes and models, meaning the bulb itself is not exclusive to the GMC S15 and is broadly available as a standard automotive component.