Goodman package unit has had blower motor changed now is not blowing in heat mode plug is unatteched fan runs on high speed how can I wire it to run on low from the plug

If the blower motor has been changed why was it not hooked up correctly to start with?

Usually when the motor is changed the plug as you call it is cut off and the motor wired directly.You can wire the motor to run on low speed if you want. The motor wiring diagram is on the side of the motor and the unit wiring diagram is on the inside of the blower door. Just match up the diagrams and wire accordingly. Or you can cut the wires at the plug and use the same diagrams to wire it using the plug itself as a connector.