How do i replace the heater core for a 1994 pontaic sunbird

i assume the shop was talking about heater core being bad and even if heater core is bad they should be able to test for a head gasket being bad actually you can totally bypass the heater core so there no reason to fix that before you check for a head gasket being bad
and if you have run for three months without coolant you will be extremely lucky if the head gasket not blown or head or block cracked
i would do nothing about the heater until car is checked for blown head gasket
honestly i would take the car some where else because they are either trying to screw u or have no idea of what there doing

it is under the dash in the center of the dash.remove the heater hoses from underneath the vehicle,remove the sound insulaters from under the dash,remove the air duct,and remove heater core access panel screws, remove heater core straps,remove heater core.

You should get a Haynes manual. You have to go on the inside of the vehicle and remove the panels, to get to the heater core. Then you have to jack the vehicle up to remove the heater core from the hoses. This is a tedious job and takes time. But not to difficult. I'm not a mechanic, and it took me about 4 hours.

get a mechanic to have a hands on look ,could be a multitude of problems from head gasket to cooling fan

This is a fairly easy fix. Disconnect the heater hoses under the hood. Go to passenger side under the dash remove the lower panel from the dash to access the heater box. Unplug any and all connections on the lower half of the heater box and remove the lower half of the box should be about six screws you should see the heater core and the evaporator core. Heater core skinnier of the two should have one or two straps holding it in remove the straps and remove the core and replace.

check heater hoses,they go from back of motor to heater connections on firewall.if you replace them,split old hose[s] with a razor knife & cut away.pulling or twisting on heater core connections may damage them.

REMOVE OR DISCONNECT

1. Negative battery cable and drain cooling system.
2. Raise vehicle and remove drain tube from heater case.
3. Heater hoses from heater core and lower vehicle.
4. Right and left sound insulators and steering column opening filler.
5. Floor air outlet duct and heater core cover.
6. Heater core mounting clamps and remove heater core.

INSTALL OR CONNECT

1. Heater core and heater core mounting clamps.
2. Heater core cover and floor air outlet duct.
3. Right and left sound insulators and steering column opening filler.
4. Raise vehicle and connect heater hoses to heater core.
5. Drain tube to heater case and lower vehicle.
6. Negative battery cable and fill cooling system.
7. Check for leaks.

Remove all plastic duct under dash bord till you find core

The heater core is located there just under the dash in the plastic enclosure. AC parts may also be there. Remove the heater enclosure to expose the core. You will have to disconnect the heater hoses from the engine to remove the core and enclosure. Then inspect the core for the leak, usually at a line connection.
The core can then be soldered or sent to a radiator shop for repair. You should have some expertise in repair to attempt this job on your own, otherwise take it to the shop. Some people would try radiator stop leak since the heater core is part of your cooling system, but I wouldn't recommend it. Remember to replace the anti freeze when you are done.

Aside from disconnecting your heater hoses, the only fix is to replace the "heater core". The "heat" function of your climate control system relies on heated engine coolant to work. The small elbow that you describe, is most likely the condensate drain. The "heater core" and the "evaporator core(for the a/c)", are located beneath the dash assembly in an enclosed housing. The "heater core" should be serviceable without removing the complete housing as with some vehicles. I hope this helps