but seriously.... the blower motor like yelo said.. or possibly the a/c controls in the dash..

 

my bud has an 05 1500hd..one day he turned his a/c on in a big hurry. now his a/c blows no harder than #4..his #5 is out.

 

They discontinued the a/c filter after 02 or something like that....

As for the air not blowing hard, I'd think either the blower motor is weak, or possibly a blend door is stuck not allowing the air to flow in a concentrated path through the ducts that you have it set on. Check and see if air is dribbling out under the dash or through the defroster vents. I also remember an old TSB about a sealing problem with one of the HVAC ducts, don't remember the specifics though

 

ok so i was driving friday and all the sudden it kicked up and started blowing hard and then after some time it went back down. it did sound like some was coming out the defroster.

 

His problem is the connector where it plugs onto the blower motor resistor under the passenger side of the dash. The connector melts at the #5 pin, and it typically takes out the resistor too. I had to replace the resistor and harness plug on my 03 SS , I "found" one in my toolbox when I got home from the junkyard.

Dave