My 2000 Astro, 75000 miles, blows AC or Heat just fine but only through the windshield ports, not the lower ones directed at the passenger/drivers, where you want it when it's hot out. Is this a mechanical issue like a blocked port, or is it the diverter switch on the dash which selects the desired location for AC/Heat?
Thanks for your input.

Check the hard nylon vacuum line connected to the LF corner of the intake manifold. This line crosses to the right side of the engine in front of the TB. It is notorious for cracking due to heat in the engine compartment.

http://www.wwdsltd.com/files/NVanHVACVacLine.jpg

That photo shows the line traversing the space in front of the throttle body, and also shows where I replaced the deteriorated nylon with rubber vacuum tubing.

The line routes to the right side and connects to a check valve here:

http://www.wwdsltd.com/files/NVanVacSourceID.jpg

awsome pics way better than words very common problem

Had same problem on my 2001 safari, vacuum line behind condenser on passenger side connects to canister under right front frame.

My 2000 Astro had the same problem, I just patched the broken line with a 2" long piece of rubber hose. Sometimes a bigger section needs to be replaced due to deterioration of the plastic line.

BTW, Defrost is the default mode on many vehicles if there is a control problem with the doors.

Finally got around to replacing the vacuum line as suggested above, and that's all it took. Thanks.