Yes, unfortunately a lot of mappers these days are very inexperienced or not very careful about having a properly structured mod map folder.
If a mod map has no custom foliage layers added you can just create a "foliage" folder, then follow my instructions.
You then just need to edit the map.i3d file in notepad++ and search for the appropriate crop type xml filepath.
For example search the fileIDs for the file "barley.xml" and make sure instead of the $data location rename it so it loads from the mod map location instead.
That would make it "foliage/barley/barley.xml".
Hope this helps. If you need further help let me know.