2004.5 dodge 2500 stereo

I recently bought this truck which had an aftermarket kenwood stereo installed with the infinity audio system. But only the front dash speakers will work, I checked the fuse for the radio amp in the engine compartment and it is ok. I pulled the stereo out and all the wiring looks correct, does any one know what might be wrong?
Thanks

I believe the remaining speakers run off the factory amp and the dash run off the radio (I could be wrong). I would first check the remote turn on from the radio to make sure its still turning on the factory amp. Then, if that tests ok, I'd start pulling door panels and checking speakers.

Yes, You need to make sure the new radio's amp turn on wire is connected.  If the back speakers worked with the stock deck, they should work with the new one.

I cut the blue amp wire and the front speakers would not turn on. I reconnected and they turned back on. The door speakers have never worked since I bought the truck. Where is the factory amp and how can I check that it is turning on?

They had an amp in the truck previously before I bought it. So I think they bypassed the factory amp and used the aftermarket. So when they took it out the 4 door speakers had no power

The amp is behind the glove box. they probably either uplugged it or cut the wire and tapped into them. Hopefully theres enough there for you to reconnect everything