Buick GS 350 Back Up Light Bulb Size

The Buick GS 350 back up light requires a P21W bulb size for proper installation and function within the rear lighting assembly. Data for the specific bulb requirements covers the range from 1968 to 1969.

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
1968-1969	P21W

Bulb Specifications

Description	Bulb Size
ECE standard (Europe & worldwide). Single-filament signalling bulb with BA15s bayonet base. 12V, 21W.	P21W

Other Locations

Additional Notes

The P21W is the specified back up light bulb for the Buick GS 350 across the 1968-1969 model years.
The P21W is an ECE-standard single-filament signalling bulb rated at 12 volts and 21 watts.
It uses a BA15s bayonet base, which is a single-contact bayonet fitting with a 15mm collar.
The ECE designation indicates the P21W was developed under European and worldwide regulatory standards rather than North American SAE standards.
The BA15s base type is not interchangeable with the BA15d double-contact bayonet base, as the contact configuration differs between the two.