Volkswagen 412 front turn signal bulb uses 1157 size while rear turn signal bulb uses P21W size. Replacement chart covers 1973 to 1974 production years with corresponding bulb specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Volkswagen 412 uses the 1157 bulb for the front turn signal across the 1973-1974 model years.
The 1157 is an ANSI-standard dual-filament bulb rated at 12V with a 21W main filament and a 5W secondary filament, mounted on a BAY15d bayonet base.
The dual-filament design of the 1157 allows a single bulb to serve two lighting functions, typically combining turn signal and parking light duties in one housing.
The rear turn signal on the Volkswagen 412 uses the P21W bulb across the same 1973-1974 model years.
The P21W is an ECE-standard single-filament bulb rated at 12V and 21W, fitted with a BA15s bayonet base.
The BAY15d base of the 1157 and the BA15s base of the P21W are not interchangeable. The BAY15d has offset locating pins to prevent incorrect installation of a single-filament bulb in a dual-filament socket, while the BA15s has symmetrical pins suited to single-filament applications only.
The 1157 follows ANSI standards primarily used in North American markets, while the P21W follows ECE standards used in European and broader international markets, reflecting a split in regional lighting standards within the same vehicle.
Both bulbs share a 12V operating voltage and a 21W output on their primary filament, though their base types and filament configurations differ.