are you sure your car has a catalytic converter?-if so,then
the o2 sensor is located underneath the car on the the exhaust-from memory it is near the front section of the exhaust-you would need to get under the car to locate it or get it onto a ramp,
also look at page 4B.10-section 64 TO 70 of the haynes manual for 190e cars.
hope this helps.
i haven,t looked under the front carpets to see if i have a similar setup to yours ,when i do i will let you know.