GMC B7 Back Up Light Bulb Size

The GMC B7 back up light utilizes P21W and 1156 bulb sizes for the rear illumination system. Data for the specified lighting components covers the period from 1996 through 2002.

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
1996-2002	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 B7 uses the 1156 bulb size for the back up light across the 1996-2002 model years.
The 1156 is an ANSI-standardized bulb type, meaning its physical and electrical specifications are governed by a recognized North American standard shared across the United States and Canada.
The 1156 operates on a 12-volt electrical system and draws 21 watts, which is consistent with standard automotive incandescent reverse light applications.
The 1156 is a single-filament bulb, meaning it serves one lighting function only, as opposed to dual-filament types that handle two functions such as braking and running light simultaneously.
The 1156 uses a BA15s bayonet base, where "BA15s" denotes a single-contact bayonet fitting with a 15mm base width, a common base type in automotive lighting.
The BA15s base is a single-contact design, which is electrically incompatible with the BA15d double-contact bayonet base used on some other bulb types despite the similar physical appearance of the two base styles.