GMC K15 Back Up Light Bulb Size

The GMC K15 back up light uses a 199 or 1156 bulb size for replacement applications. The replacement chart covers 1973 to 1978 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
1978	199
1976-1977	1156
1975	199
1974	1156
1973	199

Bulb Specifications

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

Other Locations

Additional Notes

The GMC K15 back up light specifications cover model years 1973-1978, with two bulb types specified across those years: the 199 and the 1156.
Both the 199 and 1156 are ANSI-standardized bulbs recognized in the US and Canada, each operating at 12 volts with a 21-watt output and a single filament.
Both bulb types share the BA15s bayonet base, a single-contact base design where the "s" denotes a single pin offset to one side of the base.
The 199 is a variant of the 1156, meaning the two share the same base type, voltage, wattage, and filament configuration.
Despite their technical similarities, the GMC K15 specifications do not treat the 199 and 1156 as interchangeable, as each year carries a distinct bulb assignment rather than listing both as acceptable fitments.
The 1156 is specified for the GMC K15 in 1974, 1976, and 1977, while the 199 is specified for 1973, 1975, and 1978.