So I am having a problem with my A/C that everyone says is my cabin air filter. When I am parked, the A/C hardly blows at all, but when you speed up it starts going. By the time you hit 70 mph there's a pretty good breeze going, but when you slow down it stops blowing as much. Like I said, everyone straight up says cabin air filter, easy solution. Well, I drive a 2004 Chevrolet 2500HD and as I've looked all over the internet everyone says my truck doesn't have a cabin air fitler. So how do I fix the problem if my truck doesn't have that part?

Most cabin air filters can be found behind the glove compartment. To gain access, open the glove box and pinch in the lower compartment sides to let the stops pass by the escutcheon. Proceed carefully and either be sure to remove the soft extend devices from the compartment. Once down (good to unload first), it will expose the plenum that is above the fan. There is usually a door that is easily removable by pulling the door off. And then remove the air filter. Best choice: replace it. Second choice, clean it by blowing out debris or clapping it on a hard object. Then wash it in a bucket with a bit of detergent. Everyone should do this every year for better nasal health. Older cars (depends on model but around 2000) may not have one.

There is a cabin air filter in the unit. The filter is behind the glove box. It slides out.

Can you hear the blower motor running ?
Could be a dirty cabin air filter, or blower problem, or blend door problem. The blend doors direct air thru the heater core and evaporator core as well as the vents.
If you have not replaced the cabin air filter check your owners manual on how to check it.

Have you checked the cabin air filter ? Can you tell if the blower is running on high speed ?

Have you ever replaced the cabin air filter (AKA pollen filter) I would suggest you do that and see if it cures the low air flow problem. If the problem persists then you may have an obstruction at the cabin air intake area like a plastic bag.

the cabin air filter is located on the passenger side of the cowl. after opening the hood, remove the windshield wipers and the black covering ( 2 small T-30 screws, and a few body grommets). after you remove the cowl there will be a grey accordion looking filter in the cowl, remove that and replace with the new filter.

do all of the fan speeds work? if not you need to replace the blower motor resister. If all of the fan speeds work then you need to clean or replace your in cabin air filter. When this filter gets clogged you get very little air flow from your vents. Instructions for your cabin air filter service are in your owner's manual

it does not have a cabin air filter. fresh air is drawn in through the wire mesh at the base of the windshield. make sure this is unobstructed.

Just to the right and above the accelerator pedal you need to remove the left sound insulator panel. Then remove the filter access cover by pushing down and then pulling out. It kinda pops off. Remove the tape on the first filter and then remove the filter by pulling the filter tab. Remove the tape on the second filter and then remove second filter by pulling the filter tab. Remove the third filter by pulling the filter tab. I know you didn't ask about all that but just in case you wanted to replace them.

your cabin air filter is located under the windshield wiper cover on the passenger side of car open your hood to see the access panel