White/GMC WIL Back Up Light Bulb Size

The White/GMC WIL back up light requires a 1156 bulb size for proper installation and function within the rear lighting assembly. Data for the specified component covers the period from 1988 through 1992.

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
1988-1992	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 1156 is the sole bulb type listed across all model years in these specifications, covering the White/GMC WIL from 1988-1992 for the back up light position.
The 1156 operates on a 12-volt electrical system and draws 21 watts, placing it in the mid-range of standard automotive incandescent output.
A single filament is the defining electrical characteristic of the 1156, meaning it serves one lighting function only, which aligns with its application in a dedicated back up light circuit.
The BA15s bayonet base is a single-contact fitting, consistent with the single-filament design. This base type requires a quarter-turn locking motion to seat the bulb in its socket.
The 1156 carries ANSI standardization recognized in both the United States and Canada, indicating the bulb conforms to a defined set of dimensional and performance tolerances.
The BA15s base is not interchangeable with the BA15d base. The BA15d is a dual-contact bayonet fitting used on twin-filament bulbs such as the 1157, and substituting one for the other is not physically compatible.