'02 Blazer heater fan only works on 2 and 3
 

but not 1 and 4??? any ideas guys???

sounds like blower motor resistor...it mounts just to the right of the blower motor if your standing in front of truck.
if you do a search on it you may find a how to and some info to verify before buying a part.

thanks for the reply sir:)

Hi doesn't go through the resistor.
Check your underhood fuses, there's a 30a for just high speed. And a relay that's actuated in "4".
Not running in "1" would be the resistor
That or the speed switch is faulty

Thanks for the correction...i learned soemthing on this one too.
:D