To be certain, you should:
* Consult your owner's manual: This is the best resource for precise location and instructions.
* Look for a large rectangular or cylindrical housing with a noticeable air intake tube connected to it. This housing usually has a few clips or screws securing a top cover. The filter is inside.
If you can't find it using the owner's manual and visual inspection, a quick online search for "[Your engine type] 1999 Dodge Ram 3500 air filter location" should yield images and instructions. Remember to replace your engine type with the actual engine found in your van (e.g., 5.9L V8, etc.).