Heater vents behind the front seats on 2003 GMC Safari

The 2-vents (1-each) on the floor directly behind the driver and passenger seats are "not" blowing any hot air when both the front and back fan blowers are on high. "All" other vents are blowing air through them except those 2....HELP!! I've had it in the shop 3 times.

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1 answer 2 comments
Are the vents in question blowing "no" air or "cold" air when they should be blowing "hot" air?
no air...and thanks much for your response.
Also, to clarify...the vents in question not blowing any air when all the others are blowing, are the 2-on the floor directly behind the driver and passenger seat facing to the back of the van, not the one's on the side panel behind the drivers seat. THANK YOU, I look forward to your expert advice as it's driving me crazy! Lee