1999 town&country van,center light works,but two side break lights do not work.what is the problem?

do you mean brake lights ?

Start w the basics are the bulbs any good? Is there voltage to the bulbs? Try bulbs first. To extract tail lights, open hatch inside edge of the hatch frame are two Phillips screws remove both screws gently swing fixture out away from vehicle be use caution as the fixture has a front tab that can be easily broken.