The boy has a 2015 Sportster 48 that he wants to change out the speedometer with a MotoGadget one. I have asked the question earlier as to how to wire up the unit but all attempts failed. Does anyone here know what wire in the harness that comes from under the tank and up to the unit that actually controls the speedometer function?

 

The twisted pair of red/white & black/white wires in the sub loom running up to the speedo are the ones that provide information to the speedometer. Your son's bike uses something called CAN communication on a CAN bus system over these two wires. I believe HD's version is called HD-LAN and was introduced onto Sportsters from 2014.
You need to check that the MotoGadget speedometer you are trying to fit is compatible with the above.