Something must not have went back together correctly when replacing the heater core. Looks like someone has to do the job over again.
blower motors bad check fuse bore replacing
Motor hasseized.
A short.
Did you chech the fuse?
check for a blown fuse that says heater or blower motor
Heater blower motor is defective and locked up, or possible short in the wiring.
If the heater fan is not blowing on a 2001 Chevy S10 Pickup, it is possible that the fuse needs replacing. It is also possible that debris is interfering with the operation of the fan. Could also be, a faulty fan motor, a faulty fan motor speed resistor, a faulty fan motor switch,
The fan motor has a short...change it.
You should check the thermostat and the fuse for the heater.
Fuse, switch, speed resistor or motor.
You have a short somewhere in that circuit if the fuse blows soon after replacing it check the condition of the wires in that circuit
In a 1986 Cadillac Fleetwood, the heater fuse is typically located in the fuse box under the dashboard on the driver's side. You may need to remove a cover to access the fuses. Check the owner's manual for the specific fuse number related to the heater for accurate identification. If the fuse is blown, replacing it should restore heater functionality.