Hope you and Karin are ok from the wreck. Assuming you are if you're thinking about changing a heater core. The factory never installed a filler or bleeder port on the heater coolant lines as there was no need for it. It had to be installed by a previous owner or service shop.
The Fiero Store sells a heater core bypass, but that will take time for you to get. A piece of copper tubing shaped into a U can be used by disconnecting both hoses in the front compartment then diverting one into the other. The hoses are different sizes, so it might be a little hard to get one hose clamped really well. It may be easier to get some scrap PVC 1/2 and 5/8 piping at your local building supply with a reducer and elbow to build a diverter.
Replacing the core isn't hard. There are only 5 screws holding the cover in place. You can look for the coolant leak in the left front of the passenger footwell. If you have mats it may not be so readily visible. I just replaced mine last week when it began pouring on my granddaughter's foot. By that time it had been leaking down the side of the tunnel and my whole floor was saturated.
Pull the hoses off the heater core nipples in the front compartment. Get a big bath towel to cover the passenger floor before removing the cover so you catch any coolant in the cover. Also to dry out the heater box when you remove the core. You will need to remove the right front speaker to access a screw that's on top of the box. I leave that one out when I reinstall. Don't leverage against the speaker grill opening to pry the grill out. The dash breaks easily. Place something solid at the edge and leverage on it or use a strong dental pick to lift the grill out until you can get it with your fingers.
Remove the flasher/chime mount and just let it hang on the harness. The other screws are around the perimeter of the cover. Remove them.
Loosen the screw that holds the core retainer band at the left end of the core and you can remove it from the inside. I would recommend reaching into all areas of the box to remove any debris like leaves and acorns that may have been pulled in or brought in by chipmunks and the like.
All current replacement cores are 1/4 to 3/8 inch shorter than factory. Insert the core into the box and securely clamp it with the retainer band at the left end. This is the only retainer for the core except for the metal springs on the inside of the cover. The band clamp is the most important thing to have in place.
Install the hoses, replace the cover and the 4 screws, leaving the top screw out. It really isn't needed and will make the next replacement much easier, if ever needed.
Rock Auto has the cores, both in aluminum and copper.
https://www.rockauto.com/en/catalog/pontiac,1988,fiero,2.8l+173cid+v6,1249208,heat+&+air+conditioning,heater+core,6864