I ecently purchased a 2008 3500 Express and the previous owner said that the key fob dosen't work but that it might be the battery,

First of all, I don't think he told me the whole story, but when you buy a vehicle on a private sale, you gets what you gets.

I replaced the battery but it still doesn't lock or unlock the doors, but it does turn on the headlights when you push the center button. I didn't try the panic button because it's only 5:30AM here.

Does anyone have experience or knowledge in these matters? Thanks, Jim

 

Never mind!!! There are many YouTubes on this topic. First of all I had to open up the fob and clean the contacts. Looks like someone had spilled a soft drink in there.
Then it needed re-programming. After those steps it works great. Thanks, Jim