Rick,
Not sure about the 04 but I added one to my 2000 Marquis. I bought mine on Amazon for about $250. IMHO they are cheap insurance. The cost and hassle of a blown inverter is a real bummer. (You get a real sick feeling when you plug in and hear a buzz and then smell burnt circuit board). I opted for the hard wired unit. Installation is easy and you don't have to worry about theft, rain, etc.
Steve