1999 Dodge Dakota A/C-Heater Vent problem

I have a problem with my A/C-Heater vents. My wife drove the vehicle to Colorado a few weeks ago and about 200 miles north, she called me to tell me that the air conditioning had stopped. The fan motor was still working but no air was coming out of the vents. She also said that there was no air coming out of the defroster vents either. She finished her trip and a couple of days later she called to tell me that it started working again. I haven't had a problem since then, everything seems to work fine. I thought about the vacuum check valve, but I can't locate the valve. Do you know of anything else that could cause the problem and if you still think it might be the check valve, can you tell me where I can locate the unit?