Heater/AC selector knob help!

We have a 1998 Venture van and the knob that controls the speed of the air on the dash!It only works on the 4th speed all of the others have either burnt out or just doesn't work! Can anyone give suggestions or remedies! Thanks in advance!

Place ignition switch in run, blower on low setting. Backprobe terminal B of the blower resistor (yellow wire) to ground with a test light. The resistor is what the fan plugs into, lower right under dash, it has two connectors going to it. If the test lamp lights up, replace the resistor. If it does not light up, replaced the control head. You know the fan is ok.