2004 Mitsubishi L200 interior heating non existent,

I dont know the heater set up on that year / model but usually it's a case of checking the hoses going into the heater matrix and seeing if both hot . Then if it has a heater control valve , check it is opening . If no heater valve and heat distribution is controlled by flap , got to check actuator or cable is moving the flap . Often on some cars if you can get under dash and see how it operates you can sometimes manually operate any exterior linkage to move the flap .