1999 GMC C1500 jsmoke_07

Here is the removal and install for your truck
1991-92 R/V and 1991-98 C/K Series

Disconnect the negative battery cable.
Drain the cooling system.
Remove the instrument panel glove compartment.
Tag and unplug all electrical leads as necessary.
Remove the center air distribution duct from the floor.
If necessary, remove the ECM and mounting tray.
Remove the hinge pillar trim panels.
Remove the blower motor cover and then remove the motor.
Remove the steering wheel and tilt back the instrument panel slightly.
Remove the coolant overflow tank. Disengage the heater hoses at the core connections.
Unfasten the attaching screws and nut and then remove the heater case.
Remove the seven screws and lift off the heater case cover.
Remove the fasteners and brackets that hold the heater core to the case and lift the core from the case.
To install:

Install the heater core in the case.
Install the heater case bottom plate. Make sure there is a good seal.
Install the heater case into the truck. Tighten the 4 lower screws to 17 inch lbs. (2 Nm); the upper screw to 97 inch lbs. (11 Nm); and the nuts to 25 inch lbs. (2.8 Nm).
Connect the heater hoses at the core tubes.
Install the coolant overflow tank.
Install the instrument panel and the steering wheel.
Install the blower motor and cover.
Install the trim panels and if removed, the ECM. Install the air distribution duct and reconnect all electrical leads.
Install the glove compartment. Fill the cooling system and connect the battery cable.