99 F250 superduty blower issue

Ive been driving this truck now for about 10 years and have noticed that the last couple years the air speed out of the dash vents is slowing down to where I can barely feel it blowing on me. I have checked the squirrel cage under the hood to be sure it was clean and spinning well and it was. I also removed the glove box and cover behind it and all looks clean and clear there also. I have done some research and cant get a solid answer as to whether or not this year model should have a cabin filter. If it should, where is it located?, and if not, where does the vent system suck air in at so I can check for obstructions? Its easy to tell that the blower is blowing the right speed because you can hear it roaring.....just sounds restricted. Also, as a side note, no matter what setting I have the selector on, they all sound restricted. Any help figuring this out would be greatly appreciated.

Even though no one has replied yet, I wanted to add that the blower motor suddenly went out on me yesterday so I changed it out. Even with the new motor the air only trickles out the dash vents on the highest setting. I should add that when I removed the bad motor I glanced into the opening and could see clearly all the way to the floorboard inside the cabin so it isn't restricted at that point. Going to do some research on how the blend doors operate and their location to see if maybe that's what the problem is. Its just strange how all the settings work, but the air flow out the dash is so minimal. :frown: