A/C blower motor intermittent

I have an '03 GMC Safari that has front a rear air conditioning. On occasion the front blower motor will not come on. I have never had it quit after it has started. It only seems to do it when I start the vehicle and after a short time of driving it will just come on and start blowing. I have a neighbor who has an '01 Astro that does the same thing. Any ideas? The blower is fine. It seems to be a relay switch or something to do with the dash controls of the blower motor. Is there a relay that goes bad or am I looking at replacing the switch on the dash?

My '97 had a blower problem that was the speed control board with the resistors and a relay.
The GM sealant ate the printed copper wires on the board.
The board is on the blower box, behind the overflow and washer bottles in the engine bay.
You can check that over. I was able to solder a wire over the bad parts and all speeds work.
Also, there are 2 fuses ('97 anyway). 1 for low speeds, and a 2nd for high.
And a friends newer van (2005-6?) has an intermittent blower motor. If he hits it with a hammer, it will run for a while.
Hope that helps

If blower comes on when you tap it-- then this is a sign the blower is worn out

if blower does not come on when requested comes on when you hit a bump- then replace the blower motor....

I believe the blower is also controled through the ignition switch in the steering collum