After engine is warmed up set heater temp for HOT and pop the hood, feel both heater hoses, if both are hot then your problem is the Blend Door inside the cab.
If neither or only one heater hose is hot, then either the new core is blocked or the control valve isn't working.
Remember, "new" no longer means "it works", "new" now means "never tested"
Google: 2002 Ranger Blend Door
It is a common issue.
2002 and up are similar, and for pretty much any Ford truck