Welcome, I am Chris (aka-Moose)
"FORD F150 1989 4X4 4.9 ENGINE,BLOWER MOTOR WORK JUST IN HIGH SPEED. I REPLACED THE RESISTOR."
Normally this is the resistor to blame but its new, so we need to diagnose the switch.
At the switch or resistor, on low neither the LG/WH or YE/RD should be ground
On speed 1 the YE/RD should be ground
On speed 2 the LG/WH should be ground
If you do not get these results, then the blower switch is bad.
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