A/C Fan Intermittent

I can usually handle the basics on a car...oil change, tune up, tires, brake change, and belts... but this one has me a bit puzzled.

My A/C fan is now only working intermittently...sometimes never, other times I can pound on the dash above the center vents and the fan starts blowing. The air is cool and turning off the recirculate button will usually allow enough outside air to blow through to cool down the vehicle (as long as the car is in motion). I'm tempted to tear the dash apart and take a look but I'm afraid it may show me nothing and I'll have to put everything back together and take it to a shop.

Any ideas? A loose connection? Short with the fan?
Where should I start in troubleshooting?

The blower and resistor relay pack are behind the glove box, usually the relay/resistor fails but you should check for 12 v into the resistor and current out to the blower.