Nothing will blow from the center vents
 

I am working on a 04 rainier with front a rear ac, from what I have been told the Ac compressor is bad when it try's to start up it practically stalls the engine. So I am assuming its a seized compressor or starting to at least.
but the big problem is I can get it to blow from the defrost windshield area but I cant get it to blow from the center vents at all. heat or air. I have replaced the blower motor thinking maybe it was weak.

I can get a slight breeze from the small round vents on the outer dash by the door but nothing I the vents in the middle. and with AC that's sorta where you need it.
Any idea on what it could be or what I should check?

Mode actuator? Vacuum working?

that's what I was thinking on the vents as well.
but it started to do something weird the other day the compressor wouldn't turn on properly but when I jumped it it came on fine, but without the jumper when it comes on it nearly stalls the engine

We were working on the vents.