GMC C25 Back Up Light Bulb Size

GMC C25 back up light bulb utilizes 1156 and 199 size bulbs for this truck model. The replacement chart covers vehicle production spanning 1973 to 1978 with corresponding bulb size data.

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
1977-1978	1156
1976	199
1973-1975	1156

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. 12V, 21W.	1156
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. Variant of 1156. 12V, 21W.	199

Other Locations

Additional Notes

The 1156 is the most frequently specified back up light bulb across the GMC C25 lineup, covering the 1973-1975 and 1977-1978 model years.
The 1156 is an ANSI-standard single-filament bulb rated at 12 volts and 21 watts, fitted with a BA15s bayonet base.
The BA15s bayonet base uses a single contact pin, which mechanically locks the bulb into its socket with a push-and-twist motion.
The 1976 GMC C25 takes the 199 bulb rather than the 1156 used in surrounding model years.
The 199 is classified as a variant of the 1156, sharing the same ANSI-standard BA15s bayonet base, 12-volt rating, and 21-watt output.
Because the 199 and 1156 share the same base type and electrical ratings, the physical socket interface is the same across all six model years covered in these specifications.
Both bulb types carry ANSI recognition valid in the United States and Canada, indicating conformance to a standardized set of dimensional and performance criteria.
Single-filament construction means each bulb serves one lighting function only, with no secondary filament for a reduced-brightness mode.