The Subaru RX uses the 9004 bulb across all listed model years (1985-1989) for both high beam and low beam headlight positions.
The 9004 is a dual-filament halogen bulb, meaning a single bulb unit contains two separate filaments to serve both the high and low beam functions within one housing.
Because the Subaru RX uses the 9004 for both headlight positions, the high beam and low beam sockets accept the same bulb type, though each position draws on a different filament and wattage rating.
The 9004 operates on a 12-volt electrical system and carries a wattage rating of 65W on the high beam filament and 45W on the low beam filament, reflecting the greater light output required for high beam use.
The 9004 bulb follows the ANSI standard recognized in the United States and Canada, and it is fitted with a P29t base, which is a specific bayonet-style connector that determines physical and electrical compatibility with the socket.
The P29t base is not interchangeable with other common halogen base types, so substituting a bulb with a different base configuration would result in an incompatible fit for the Subaru RX headlight sockets.