Replacing a heater core on a 93 Toyota Pickup?

If the coolant flow is hot both in and out then you have a blend door problem. The doors route the cooling/heat into the cabin. Looks like you will have to pull apart part of the lower dash to find the problem. The doors are operated either by vacuum or it’s electrical. You probably need to pick up a repair guide like Haynes for your vehicle. They are sold at most large auto parts stores or check out one at your local library.