GMC K25 Back Up Light Bulb Size

The GMC K25 back up light bulb size chart specifies 1156 and 199 bulb sizes for this vehicle model. The replacement chart covers 1973 to 1978 production years 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
1975-1976	199
1973-1974	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

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Additional Notes

The 1156 is the most frequently specified backup light bulb for the GMC K25, covering the 1973-1974 and 1977-1978 model years.
The 199 bulb is specified for the 1975-1976 model years of the GMC K25 backup light position.
Both the 1156 and 199 are ANSI-standardized bulbs recognized in the United States and Canada.
Each bulb type operates at 12 volts with a 21-watt output, placing them in the same power class.
Both bulbs use a BA15s bayonet base, which is a single-contact bayonet fitting with a 15mm collar.
Each bulb carries a single filament, meaning it serves one lighting function only, with no dual-mode capability.
The 199 is classified as a variant of the 1156, indicating a close relationship in design and electrical characteristics despite carrying a different ANSI designation.
Because the 1156 and 199 share the same BA15s base and identical voltage and wattage ratings, physical fitment between the two is closely related, though the variant designation warrants attention when sourcing a direct replacement.