As I recall it takes an 18 mm wrench but that won't work because it has a stamped metal tab on it already to act as a wrench. The tab will likely bend or break off if you attack it with an impact wrench. Just turn the bolt slowly by hand with a ratchet. If it feels like isn't going to come free, spray some Mopar Rust Penetrant in the hole where the nut lives and let it sit for an hour or two. Don't waste your time with other products except maybe PB Blaster which I've heard good things about. And you don't want Mopar MP-50 either. That is similar to WD-40. Mopar Rust Penetrant is black and bubbles and will do in ten minutes what WD-40 will do in a weekend. What you must do though is to wash that material off when you get the parts apart. If you leave it on threaded parts it will create a space for water to sneak in which will make those parts much harder to separate next time. If the metal tab is bent and spinning you'll have to catch it and hold it with a pliers or Vise Grip. If it is broken free you're going to have to use a wire feed welder to reattach it. The heat from welding will make the nut come free easier if you don't wait for it to cool down.