how to replace the backup sensors on a 2003 lincoln?

https://www.youtube.com/watch?v=uf4RbK9f1ko

http://repairpal.com/back-up-sensors-not-working-327

http://www.justanswer.com/ford-lincoln/6uiy6-lincoln-platium-04-towncar-backup-sensors-not-working.html#re.v/265/

Have you had your car washed then detailed lately? Or often? If so most common cause is a build up of wax around the sensor . Inspect the sensors for any residue buildup and clean if needed. Otherwise you will need access to a scan tool to monitor the sensor input to see if any are reading. the sensors themselves do go bad and may need to be replaced as long as there is nothing obstructing them.