GMC P4500 Back Up Light Bulb Size

The GMC P4500 back up light utilizes a single 1156 bulb size for illumination across all listed vehicle applications. Data for the specified component covers a range from 1979 through 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
1979-1990	1156 (Medium Duty)

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. 12V, 21W.	1156

Other Locations

Additional Notes

The back up light bulb for the GMC P4500 (Medium Duty) is the 1156 across all listed model years from 1979 to 1990.
The 1156 is an ANSI-standardized bulb recognized in both the United States and Canada, operating at 12 volts with a 21-watt output.
As a single-filament bulb, the 1156 serves one dedicated electrical function, which in this application is the back up light circuit.
The 1156 uses a BA15s bayonet base, a single-contact mount where the "s" denotes a single pin, distinguishing it from double-contact bayonet variants.
The BA15s base type is not interchangeable with the BA15d base, which carries two contact pins. Substituting a double-contact bulb in a single-contact socket is not compatible.
The 1156 is also sometimes referenced by the trade number 7506 in certain markets, though both designations refer to the same ANSI-specified bulb with identical electrical and base characteristics.
The Medium Duty classification noted in the specifications indicates these fitments apply to the P4500's medium-duty vehicle configuration rather than a light- or heavy-duty variant.