GMC K25 Suburban Back Up Light Bulb Size

The GMC K25 Suburban back up light bulb size chart specifies 1156 and 199 bulb sizes for this vehicle model. The replacement chart covers 1973 through 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
1976	199
1975	1156
1974	199
1973	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 GMC K25 Suburban backup light specifications cover model years 1973-1978, with two bulb types used across that span: the 1156 and the 199.
The 1156 is an ANSI-standard single-filament bulb rated at 12V and 21W, fitted with a BA15s bayonet base. It is the more frequently specified of the two types, appearing in the 1973, 1975, 1977, and 1978 model years.
The 199 is also an ANSI-standard single-filament bulb rated at 12V and 21W, sharing the same BA15s bayonet base. It is specified for the 1974 and 1976 model years.
The 199 is classified as a variant of the 1156. Both bulbs share identical voltage, wattage, and base type, which reflects their close technical relationship within the ANSI bulb family.
The BA15s bayonet base is a single-contact fitting. The "s" in BA15s denotes a single contact point at the base, distinguishing it from the dual-contact BA15d configuration used in other bulb types.
Because the 1156 and 199 share the same base type and electrical ratings, their physical and electrical compatibility is closely related, though the specifications assign each to distinct model years rather than treating them as interchangeable across the full range.