^^ there is a rubber plug underneath the brake booster.

Fram oil filters are poor, but their air filters are actually pretty good. 20,000 miles is a reasonable amount of mileage for an air filter, depending on your driving conditions. I use a WIX, and change it every 15,000 miles. By the way, have you done any other maintenance besides oil changes?

no just oil changes, air filter change and engine shampooings. that's it so far.
Next I want to change the air cabin filter if I ever find where it's located.

I dont think there is one.

There is no cabin air filter. You seriously need to dig out your owners manual, you're behind on maintenance. Just off the top of my head - fuel filter, transmission, coolant, differential ...

I don't have my manual, my truck was broken into a year ago and they stole everything in the glove box, including the owners manual.

You can probably get one for really cheap somewheres.

Wow thats so wrong . And you need to have it tuned up , and everything mentioned above . I change mine every 3K miles or 4 months wich ever comes first as I drive in sandy dusty conditions alot .

at's the way to go right there, 2 of the best brands for your engine

102000 miles and it runs like new.