Top front air vents not blowing

My A/C works fine but all of sudden, recently, the air stopped blowing through the top front vents. It is blowing out the bottom and the top somewhat, but not the top front main vents. Cooling is far less effective this way so I want my main vents working again. When I state this condition in a basic search, every answer that comes up is related or similar but not the same as my situation. Oh, and some say to check vacuum line on firewall others say I have no vacuum lines on a 1999. Very confusing. So what do I check for since my front vents quit blowing? Also, as far as I can tell, the vent position knob doesn't seem to work. Yes, I tried all positions and it just kept blowing the same, ie, out the floor vent and the defroster vent. So is it electrically activated or vacuum or what? It's a 3.1 six. The hot weather is coming on fast here in phoenix and I must have those main vents working.