GMC C6000 Back Up Light Bulb Size

Additional Notes

• The GMC C6000 uses the 1156 bulb for the back up light position across all listed model years from 1979 to 1990.
• The 1156 is an ANSI-standardized bulb recognized across the United States and Canada, meaning it conforms to a defined set of electrical and physical specifications shared by all compliant manufacturers.
• The 1156 operates at 12 volts and draws 21 watts, which is typical of a single-filament incandescent bulb used in a dedicated, single-function lighting position such as a back up light.
• Because the 1156 is a single-filament bulb, it serves only one lighting function, distinguishing it from dual-filament variants like the 1157, which handles two functions within a single housing.
• The 1156 uses a BA15s bayonet base, where "BA15s" denotes a bayonet fitting with a 15mm collar and a single contact pin, which physically prevents substitution with dual-contact bayonet bulbs without modification.
• For the GMC C6000, the 1981-1990 model years carry a Medium Duty designation alongside the 1156 specification, indicating a vehicle classification relevant to fitment context within that production period.
• The BA15s base is not interchangeable with the BA15d base, as the latter features two offset contact pins. Installing a BA15d bulb into a BA15s socket is not possible without altering the socket or bulb base.
• The 1156 bulb is also sold under trade designations such as P21W in international markets, though the ANSI 1156 designation applies specifically to the North American standard referenced in these specifications.