The rear light acting like that is normal when you have a front bulb out on the same side.
Double check with a new front bulb and if it's still not working, you need a new front socket. They get corroded sometimes. Just cut and splice.
Take a photo of the socket, show the dude at Ford, autozone etc. they have new ones in stock.http://www.rockauto.com/catalog/x,ca...,parttype,4024
*next time use synthetic blinker fluid, save yourself future headaches.