Check the small black hard plastic tube that runs along the back of the
firewall from the heater fittings to the vacuum "T" next to the brake
booster. It is VERY common for this vacuum line to get brittle and
break. Without vacuum the vents will default to the defrost position at
the windshield. If broken place a piece of small vacuum hose between
the break. If broken at rubber fitting on the end where it goes to the
"T", drill it out and reinsert tube back into rubber and super glue.

Hope this is helpful,

TIM (Musttanguy)
PonyPerformance.com