98 gmc safari

My blower cold or hot air is only blowing against windchield on top of dashboard any idears how i can locate whats wrong?????

Had the same problem, actually many have it's a common problem with these type GM vehicals.
There is a small plastic vacum line that goes from the rear of the engine on the drivers side across the front of the engine to a t fitting on the passanger side by the ac unit. The engine heat causes this line to crack or break completely.
You'll need to remove the intake filter box completely from the front of the engine to access the area, cut the plastic lines back where they are no longer brittle and install some new rubber hose over the ends of the plastic lines.
You'll find many postings on this and many other common problems in the "Astro" board, the vans are the same, but there seems to be more people posting on that board.
Good luck.

Wow great this helped me.. after installing a new switch, or heater a/c control unit. which didnt do the trick i read this post. the switch cost over $100, the vacuum line and fittings cost $11. wish i would have found this first glad i found this site thanks for the help hey now i have a used heater a/c control unit if anyone is in need of it.. still works fine make me an offer

This works! I have a 1997 Safari and I found this surfing around. I went all last summer with my air stuck on defrost. My Van also runs smoother. This line hooks to the PCV valve for its vacume. Thanks!