What chassis?
On some, it is above the rear axle.
When you find it, copy down the make and model number.
And a very good option, particularly if the dryer is OE/15 years old if you are not real mechanical is to just replace it with a factory (as in the factory that made it, not a Chinese "made to replace" complete air dryer).
It will come with not only new desiccant filter(s), but new valves, new heater, etc.
