Are there fuses or relays I need to check?

Not much info here. Guess it depends on what type of repair you did?

Ford Man

Changed heater core, thermostat, flushed radiator.

Might be an air bubble stuck in heater core, in-line coolant control valve might be stuck & not working right. Did ya watch to see that ya got the system burped & coolant filled up correctly? Also check that you got wiring back on right. Sometimes. Ya know. Chit happens !

Ford Man

First thing I would check is the that the coolant level is correct. Take the radiator cap off, turn the truck on and crank the heat to "Max." Allow the engine to reach operating temp. And check the coolant level. It will likely be quite low. Add coolant and keep checking the heat coming from the vents, if this is the problem, the heater will work once the system is full, this also allows the system to bleed the air from itself. After that, you may have mistakenly left something disconnected when the dashboard was removed top replace the heater core. Start with the easy check (the coolant level) first and let me know.