I have a 2003 and there is one just behind the rear axle oh the left side. you will have the new style it has a inlet and outlet before 2003 they used a filter that require a adapter that one is no longer available and requires a conversion kit which is very expensive. You can get the filter you need at NAPA I use the NAPA gold #3481 about $14.00 it is made by Wix #33481 AC Delco is GF652 Fram is G3727. The Napa gold by Wix i s a good quality. There is a web site "OMEYS" that has a lot of good info for workhorse and part numbers, they also have a DIY section I used the plans there to make a trac bar which stopped the tail wag on my 37ft Pace Arrow. Hope this helps