I know the previous owner didn't do this.  He's the kind of guy that would buy a $300 crimper to do one crimp. 

So that means it came this way from the factory, or the dealer did this.   My guess is that the dealer damaged the connector while removing the stock lift pump. 

 

This is the connector for the manifold air pressure sensor.  Does anyone have an idea what kind of problem this would cause?  I mean, if the truck runs, but doesn't have this sensor data, what would the truck do? 

Corrosion issues, separating issues (as you found).  if the truck didn't have that sensor, it wouldn't fuel.  The ECM reads the MAP sensor and references the boost data as part of the fueling table.

I never crimp important connections always solder, and if I do crimp something I always put dielectric grease on wire before crimping and then use shrink tube. 

Was the truck a one owner and what year is it?

It's a 2001 2500 chassis truck.  My wife's brother is the original owner.   

 

The dealer moved the lift pump from the engine bay, into the tank.  That's the only thing I know of that isn't 'factory'.

Rules out another owner doing it. The reason i asked year is my 02 came with adifferent MAP sensor that is not available any more so I have a short harness adapter so I can plug into the available sensor. It coverts to the connector shown in your pic. I would think some were probably done the same way. Hence the splice maybe. Still shoddy work. I have done some butt connectors but used the heat shrink type with grease in them.

This kind of wiring issues will create either P0237 or P0238 error codes.