A few months ago, early summer. I noticed that my vent control stopped working, It had started blowing to the defrosters ( Default for a vacuum leak, but no noise ).
So I opened up the dash and did my thing, pulled out the AC control box, then pulled the tabs off, suddenly all the small pieces that control the vents fall out. I try my best to put it all back together. Now the Cold-hot adjuster is stuck on cold, there's a vacuum leak ( I hear hissing now ).
So my 2 questions are, is there any way to loosen the Cold-Hot adjuster so I can service the unit, or am I stuck with crawling underneath it to put it all back together?, and How do I trace the vacuum line for the vent controls? I've read about the plastic ball on the firewall, and something about it leading to it.
All help is appreciated.

'info:
1993 LE Ext. cab 3.9L

 

I'm having the same (initial) problem as you. where my heat only blows from the defroster vents regardless of button selected on the heat control unit.

As far as loosening the hot/cold adjuster, there should be a red "flag"/tab holding the adjuster cable to the unit. Detach that, and you can get the adjuster cable off of the unit.

I haven't figured out tracing the vacuum lines yet, but I'm thinking that we're pretty much stuck with crawling under the dash and quite possibly dropping the heater core box that the vacuum lines attach to.

 

The good news is, once you drop it, you'll kick yourself since the vacuum lines route out the bottom of the box, and you can reach them with the box installed.

What you end up doing is crawling in there, laying on the front seat with your head up under the dash, and a bright flashlight so you can see the (for 1988) black and grey hoses, then follow them up to the dash and behind the radio and the control head.

HINT: Remove the radio. It's a LOT easier to see the joint where the control head hoses fasten to the rest of the hoses with it out.

RwP