truck: 1989 dodge ram, std cab, short box. how big of a job

Hi thank you for your question hope this helps.

1989-93 Pick-Up and Ramcharger

Heater Core
REMOVAL & INSTALLATION

WITHOUT AIR CONDITIONING

1. Disconnect the negative battery cable.
2. Drain the cooling system. Remove and plug the heater core hoses.
3. Remove the right side cowl trim panel, if equipped. Remove the glove box assembly. Remove the structural brace through the glovebox opening.
4. Remove the right half of the instrument panel lower reinforcement and disconnect the ground strap.
5. Disconnect the control cables from the heater housing and the blower motor wires on the engine side.
6. Remove the retaining screw between the package to cowl side sheet metal.
7. Remove the heater housing retaining nuts on the engine side of the heater assembly and remove the heater housing assembly.
8. Remove the heater housing cover retaining screws and the mode door crank. Separate the cover from the housing.
9. Carefully lift the heater core from the heater housing.
10. Installation is the reverse of removal. Refill the radiator.
11. Connect the negative battery cable, run the vehicle until the thermostat opens, fill the radiator completely and check the operation of the heater.
12. Once the vehicle has cooled, recheck the coolant level.

WITH AIR CONDITIONING

1. Disconnect the negative battery cable. Properly discharge the air condition system. Drain the cooling system. Disconnect and plug the heater hoses and the refrigerant lines.
2. Remove the condensation tube from the housing.
3. Move the transfer case and gear shift levers away from the instrument panel, if equipped.
4. Remove the right side cowl trim panel, if equipped. Remove the glove box and swing it out from the bottom.
5. Remove the structural brace from the through hole in the glove box opening. Remove the ash receiver.
6. Remove the right lower half of the dash reinforcement by removing the retaining screws holding it to the instrument panel and to the cowl side trim panel.
7. Disconnect the radio ground strap, if equipped. Remove the center and floor air distribution ducts.
8. Disconnect the temperature control cable from the assembly and tape it out of the way.
9. Disconnect the vacuum lines from the extension on the control unit and unclip the vacuum lines from the defroster duct.
10. Remove the wiring connector from the resistor block. Remove the blower motor electrical connector from the engine side of the assembly.
11. Disconnect the vacuum lines on the engine side and make sure the grommet is free from the dash panel.
12. Remove the retaining nuts on the engine side. Remove the screw that retains the assembly to the cowl side of the sheet metal.
13. Remove the assembly from the vehicle.
14. Remove the vacuum actuators, door crank levers, evaporator case cover retaining nuts and screws and the heater core retaining screws. Lift the cover off of the assembly and remove the heater core from its mounting.
15. Installation is the reverse of removal. Tighten all components securely.
16. Evacuate and recharge the air conditioning system. Refill the radiator.
17. Connect the negative battery cable, run the vehicle until the thermostat opens, fill the radiator completely and check the operation of the entire climate control system.
18. Once the vehicle has cooled, recheck the coolant level.

Installation is the reverse of removal.

Hope this helps

Good luck