How do you change the cabin air filter in a t 800 kenworth?

There are two possible locations, depending on whether you have external air cleaners or not. If you don't, I believe they use a compatible cab filter to the T600 and T660, which is a mesh type you can actually wash out and reuse if it isn't damaged. Open the hood, and, on the passenger side against the firewall, immediately past the hood, you'll see a black plastic piece with a rubber securement strap on the bottom. Undo that, and the plastic case sits in an open hinge at the top.. you can either flip it up or take it off.... the filter sits inside two plastic rails on either side of the housing.. slide the old one out and either wash it and slide it back in or slide the new one in. If you have external breathers, it's mounted to the hood under the intake tube of the right side breather. You just remove the screws for that housing, the securement plate comes off, you swap out the filters (this type is NOT reusable), and put it all back together.