Hello and welcome.
Just so iam clear. You replaced motor, resistor, and only the fan switch-not the control head?
The 2-3 speeds run from the switch to the resistor. 1 and 4 get power a little different., to switch then to resistor.
You will need to check with a test light for power at the resistor trerminal B (brown) This is low speed with fan switch on low, and mode switch on heat, or vent, etc. If no power at resistor, then need to check for power out of control head with switches in same position at terminal B(brown) on back of contol head. This circuit feeds low speed and high speed fan switch positions through a splice.
If no power ut of control head, the control head is bad. If yes power out f control head, then need to check splice. It is located in the harness behind the control head. Just follow that wires into the harness.
Let me know what you find.