Ford Super Duty (1999-2016) :: 2000 F250 - Blower Would Only Blow On High

My dad has a 2000 f250 4x4 with the 5.4. he said his blower would only blow on high. I told him it was his blower motor resistor. i thought it was under glove box. he took it apart and said no but there was a little box thing with wires he took off anyway and put back on. I then told him it was under hood below a/c lines. he replaced old one with a new one from auto zone and replaced the pigtail( plug too) fan works on high and a little bit on low very very little when u switch it to medium it stays the same. High works great. So we replaced the fan switch. (Radio was a bitch to remove) and it still does the same thing. I checked all fuses . when I check the plug at the resistor there is power to the center pin it stays on when u switch to low and medium but when u turn high on power goes out( on test meter) is there something I am missing? the blower blows on high even when u remove the plug from the resistor. Is that normal?.