I have replaced the switch, the resistor, and the fan motor but the heater fan motor will only blow high, 4 setting.
Off, 1, 2, and 3 are all equal to off but 4 blows hard.
I have not found any fuse or relay blown.
What next? 1994 Altima 2.4l
OK. You said you replaced the resistor, so I'm assuming this is the manual system, not automatic.
If the blower switch and resistor are indeed good, there must be a broken circuit -- poor connection, cut wire, etc.
Looking at the schematic, a poor connection at terminal 28 at the resistor connector, or a break at the splice leading from there to the blower motor, would produce the symptoms you've described -- nothing at 1, 2 and 3, normal high speed at 4.
We know the motor has power or it wouldn't work at all, the switch is applying the ground circuit either directly to the motor (high speed) or through the resistor for lower speeds.
It should be easy to narrow down with a test light and a schematic.
You can get the schematics and full service information at ALLDATAdiy.com.