The 1985 Chevrolet El Camino's speed sensor is located in the
transmission. It's not a single easily-accessible unit like on some later vehicles. It's integrated into either the speedometer cable housing (for mechanical speedometers) or as part of the transmission's internal workings (for electronic speedometers).
To be certain of its exact location, you'll need to consult a repair manual specific to your El Camino's transmission type (automatic or manual). The manual will show a diagram and detail its location and how to access it. Without knowing your transmission type, pinpointing its precise position is impossible.
