The two oxygen sensors in the Mercedes-Benz M104 engine are in the exhaust system, just in front of the catalytic converter and just behind it. Mercedes-Benz produced the M104 engine from 1989 to 1998. No Mercedes-Benz vehicles manufactured in 2014 used the M104 engine.
The rear oxygen sensor in the M104 engine is readily accessible, but the front sensor is mounted in a small space at an odd angle, as PelicanParts.com shows. Consumers can get help locating the oxygen sensors in their 2014 vehicles by contacting Mercedes-Benz through its website, Mercedes-Benz.com. A list of Mercedes-Benz engines, with the years in which they were produced, is available at Wikipedia.