the filter is either the first part or the last part on the exhaust. u can either push air through it, or pull it through, lets get our thinkin caps out.. If your air is exhausted out side u would want it first, because u dont want it to be visible or getting "weathered" on. there aren't in line carbon filters as far as i have seen maybe u can make one im not sure but all the active carbon is on the sides. not the middle.