It's relatively straight forward. The are 2 styles of heaters - the economy or recirculating type where the heater core and blower are contained within the cab, and the deluxe or fresh air heater that has a rather large sheet metal box on the passenger side of the firewall.
If you have the econo or recirculating type, it is easier with the glovebox removed. Simply disconnect the wiring, remove the screws holding the motor panel to the case, and lift it out. If you have the deluxe version, it's the same procedure, except the motor is on the bottom of the case which is going to make it more difficult to see what you're doing.
