Cadillac Repair: 1968 deville a/c, windshield glass, vacuum lines

Question
I can not get the blower motor to blow thru the ac vents.  It will come on and blow for the windshield de-ice mode but will not switch to vent mode.  I get a quick blast to the vents then it goes to the windshield glass

Answer
Just to clarify. You have a 1968 year vehicle ?>

In ALL cases this problem is caused by a leaking/broken vacuum line in the engine compartment usually near the carburator. Physically look and feel all the vacuum lines you can find for  leaking or being broken and repair it.