I have a 98 s10 and the other day I started it and turned the heat on and no air was blowing. I unplugged the blower motor and tested it with a meter. When the speed selector was set on 1, 2, and 3 it read 12 volts and on 4 it read zero. Then today I started it up and it now blows air but only on 1, 2, and 3.