heat/vent/ac switch

I have an 96 Jimmy that only puts air onto the windshield and floor boards at the same time. The fan speed works just fine as does the warm and cold switch. I just can't change to the vents, A/C, or any other setting. I've been told a few times that I need to trace the vaccum lines, but I have NO idea where they go past the dashboard. Any help would be apprecaited.
robts

look in the engine compartment for vac lines going into the cabin and trace them back to the engine.

some related info...
http://www.automotiveforums.com/vbulletin/showthread.php?t=394808