Hook a small light bulb or meter on the power feed to the blower motor. Then when it is intermittent see if the light or voltage is on or off.

You do not want to go digging until you have an indication of where the problem is. Could be the blower motor or the blower control circuit. You have to start somewhere.

You want intermittent. We have a newer car that on average the blower fails to run about once a year. We have owned that one for about six years now. The warranty was over before the first time. So it has done this about four times. Of course when it occurs you do not have time to check it out. By the time you do it is back on again.