We changed our battery and now the headlights and interior lights flash?

What does "nothing is working" mean? All electrical on the truck is dead or nothing is getting it out of theft mode? If everything is dead, clean and tighten the battery cable connections, then follow the smaller positive battery wire to the under-hood fuse box and be sure that connection is clean and tight. If it's just in theft mode, which it will be after disconnecting the battery without using a memory saver, unlocking a front door will deactivate it. If that's not working from the driver's door, try the passenger door or the remote key fob. When it doesn't work from one door, it's usually due to a broken wire between the door hinges, or a problem with the switch in the door.

Also check all the fuses under the hood and inside. Some often blow from the current surge when the battery is connected.

Ahh. Hold the phone. If the engine runs, it's not in theft mode. The interior lights do not flash with the alarm system but all of them do run off a Body Computer. Those don't really cause a lot of trouble on Dodges, so you might check the fuses first, under the hood and inside. They often blow from the current surge when the battery is reconnected.

I have also seen the security system cause this issue, here is how to reset the system.

https://www.2carpros.com/articles/how-to-reset-a-security-system

let me know how it goes.