1999 Ford Taurus Headlight problem

We found a deal on a 1999 ford Taurus for my daughter. New tires, brakes etc. Only problems are that she had only brights and one headlight could be changed to dim. We purchased one headlight thinking it would fix the dim problem. Well, now she only has 2 bright lights. The other issue is poor gas milage...The airfilter is new and fluids look good. Autoparts place us we need to clean the oxygen sensors.. where are they and how do we take them out to clean them? I have a '02 plymouth voyager that need this done as well. Any advice is much appreciated.