The 1816 is the sole bulb size specified across all side marker positions on the Audi Fox, covering both front and rear applications throughout the model's production years in these records.
The 1816 bulb follows the ANSI standard recognized in both the United States and Canada, classifying it as a miniature wedge type with a T3.25 designation.
A T3.25 bulb takes its designation from a tubular glass envelope measuring 3.25 eighths of an inch in width, placing it firmly in the miniature bulb category used widely in automotive marker and indicator applications.
The 1816 operates at 12 volts and draws 4 watts, consistent with the low-power requirements typical of side marker lighting rather than primary illumination functions.
The wedge base design of the 1816 means the bulb is held in its socket by the tension of the base contacts alone, with no separate metal bayonet or screw fitting involved.
The front side marker on the Audi Fox uses the 1816 bulb across the 1973-1977 and 1979 model years, while the rear side marker uses the same bulb across the 1973-1979 model years.
Because both the front and rear side marker positions share the same 1816 specification, a single bulb type serves all side marker sockets on the vehicle.
The ANSI classification of the 1816 indicates it conforms to standardized performance and dimensional tolerances, which generally allows for interchangeability among bulbs carrying the same ANSI designation regardless of manufacturer.