Heater not working on my 1992 Cadillac deville?

[RyTechGuy]

I have a 1992 Cadillac Sedan Deville V8 4.9 liter. My heater recently stopped working. It occasionally will blow hot air from the defroster vent on the passenger side. The driver side only blows cold air from the defroster and vents under the dash. The A/C never worked on the car since I bought it. I sometimes notice an clunk then a hissing noise before the blower kicks on. Im reading that this may be a heater core plugged or a blender door actuator? Should I try replacing the thermostat first? I don't think that's it? They say I may need to flush the heater core... How would I go about doing this? I know I have to remove the glove box correct? Any help is appreciated. Thanks