Not sure about the 124's, but on the 126's the O2 sensor makes the connections under the passenger side false wall in the footwell. Pull up the pass side carpet then remove the false wall. Connection is likely at the lower left side of the floor.
In the past there had been multiple discussions on drastic reduction in fuel mileage with no other problems noted. Replacing the O2 sensor restored fuel mileage to norm.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.