Honesyltly, I dont even know where to start on this but here is what I know.. my temperature slider works, my blower motor works fine because the only place air comes out is the defroster vents, so its like its stuck on defrost, BUT when I put the slider on A/C the A/C compressor kicks on...... I found some wiring issues under my glove box area...

 

If it is only blowing out of the defroster vents more then likely you have a vacuum leak. Since you have the dash open start looking at all of the vacuum lines under there and work you way to the vacuum ball behind passenger side bumper.

 

Check the grey vacuum line under the Hood on pass side it controls the vent door.and check the vac. Ball for cracks

 

Alright, I checked all vacuum lines under hood. The black line next to the grey line that comes out of the firewall was disconnected. I reconnected it, but still no change.. now, the grey vacuum line that comes out of the firewall does nothing to the lever when I change the vent settings. Is that normal?

 

The "vacuum ball" down behind the right front bumper works as a vacuum reservoir. It gets "charged" with vacuum when there is plenty. It then metiers it back out through a much smaller tube to provide vacuum while there isn't much, like when you have your foot in it.

So there are two lines up to it. On my 90 the large and small tubes are easy to see running along the inner passenger fender. The large goes clean back, across the firewall, then to the intake manifold. The small, (black on mine)(maybe "greyish), goes through the firewall to provide vacuum to the heater vent vacuum motors. I don't have cruse, but it would do that also.

If you unplug the small one before it goes into the firewall, there should be vacuum there..so there's that!

 

Its a default setting for the air to blow thru the defroster vents on the dash, when you have a vacuum leak. Try unplugging an entire section of line and replacing it with vacuum or sprinkler hose like me youll find the leak. (im cheap) or just get 20ft of hose and connectors and replace everyone of em.

I had mine leaking here

The swivel thing at the end was leaking. Got rid of it.

Good luck