GMC Topkick Back Up Light Bulb Size

The GMC Topkick back up light requires a 1156 bulb size for this vehicle model. The replacement chart covers 1990 to 2001 production years with corresponding bulb size specifications.

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
1990-2001	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 Topkick uses the 1156 bulb for its back up light across all model years from 1990 to 2001.
The 1156 is a single-filament bulb, meaning it serves one lighting function rather than combining two, such as stop and tail, into a shared housing.
It operates on a 12-volt electrical system and draws 21 watts, which is consistent with standard automotive incandescent output for reverse lighting applications.
The 1156 uses a BA15s bayonet base, where "BA15s" denotes a bayonet fitting with a 15mm collar and a single contact pin at the base.
The 1156 is classified under the ANSI standard, indicating it conforms to a defined specification recognized across the United States and Canada.
Because the 1156 is single-filament, it is not interchangeable with the 1157, which shares the same BA15d bayonet-style housing family but carries a dual-filament design and a second contact pin.