vacuum line

I just got 88 foxbody mustang gt. 5.0 with manual transmission. New to mustang's but I love them. Don't know to much about it.Aftermarket throttle body,no maf sensor,headers with x pipe,no egr valve. My pcv valve is wide open with nothing attached, and on passengers side there's a vacuum thing...

I have a problem with my 1995 GT. Whenever I turn on the A/C it only goes to the Defrost. Or whenever I put it on Floor or Vents it only goes to Defrost. Well I figured out why, On my vacuum tree the A/C nipple is blocked. There isnt a line there its just a block off. Im gonna buy some vacuum...

I need help finding out where this vacuum line leads to and which line it is. The previous owner put duct tape around it and it's very dry rotted. Thanks!