Not an easy task to do.

You are going to have to take out the lower "kicker" panels of the dash (the drive and passanger side foot well) to undo the air ducts connecting to the heater core. There are a few bolts holding down the heater core as well as a long strip that holds down the core. Undo all this and it should be able to move around a bit. Next, you are going to have to undo the coolant line that feeds into the heater core and you will prolly spill some coolant in the process(be ready to put a stopper so it won't leak). Now you are ready to remove the core (I think, I may have forgotten something up to this point). You will have to be creative to remove it, as it took me a while to do mine. To reinstall is just the proceedure backwards. It's going to be difficult to reattach the air ducts from under the dash, good luck!

If you are getting zero air (hot or cold) from the vents, then the blower motor is out. Check a connection under the driver's side panel of the center consol (the connection should be right next to the radio or the climate control) it's a plastic thing, yellow IIRC. Undo it and look at the connection, if there is some black carbon, then that is your wire break, soder it and you're back in business. If the wiring checks out (you get ~12V to the blower motor when the blower motor should be running (defrost on HI)) then replace the blower motor.

If you get cold air, but not hot air, then no heat is getting to your core, then I can't tell you what it is because it hasnt' happened to me.

Gotta love those 5000s

Joseph V.
1991 200q
1985 5000s