I changed the battery in my key fob for my 2008 Dodge Charger but it still won't unlock doors or ooen trunk.

It must be re-programmed.Two different sites, both say only a qualified locksmith or dealer can do it: http://blog.keylessentryremotefob.com/2013/04/2008-dodge-charger-key-fob-remote.html But then here is a site with instructions: http://www.fixya.com/cars/t15914982-reprogram_2008_dodge_charger_key_fob ..and a uTube that I have not watched but claims to show you how: https://www.youtube.com/watch?v=h92KqVBj_9o

Regardless, all of them agree that you must have another working fob to DIY, and Here is that one link I will put on the end to make it easy to hyperlink if you want to try their instructions: http://www.fixya.com/cars/t15914982-reprogram_2008_dodge_charger_key_fob