How do I troubleshoot my heater?

[Garp2014]

My 1998 Explorer Heater fan works intermittently. When it works it works on all speeds as normal, but then next time NOTHING! Not a whimper out of it. How do I troubleshoot this? Where do I start? Should I suspect a switch? Or a relay? Or a resistor? Or all three?

 

[stress301]

Check for power at blower motor. If no power at motor, check for power at blower resister which is mounted next to fan motor.You should have power on two wires,one coming from switch the other going to fan.If you have power going to resister ,the resister is bad

 

[Garp2014]

Thanks, but the problem is that it is an on-again, off-again situation. But I'll put my meter on it and see what I can see.

Garp2014