1996 Chevy Astro heater/air conditioning fan

Hi:
It could be a couple reasons. The switch may be bad itself, a blower motor resister, or the motor itself. All you can do is when it stops working, start at the motor to see if you're getting power. If you are, the motor must be bad. If no power, check for power to the resister, if power is there, replace the resister (most common problem). If there is no power to the resister, check the switch to see if there is both power in and out of the switch. If there is no power out, replace the switch.

I wish there was an easier way, but that's what you need to do.

Joe