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My Dad just gave me a WeatherGuard toolbox out of one of his old work trucks. It's from '93 and had a hard life. It had diesel fuel with caked on dirt all over the exterior. Assorted grease, oil, and other mystery substances all over the inside. I cleaned all that off with Simple Green. Man, I love that stuff! Use it full strength, and it'll clean anything! After that, I used The Intiminator aluminum polish. Stuff works miracles! I use it on my big trucks to get them ready for shows. I get it at my local Peterbuilt dealer. But, I've seen it at a couple of other dealers and truck stops. For a bit of extra shine, I took the buffer to it! Looks better then new.
If your aluminum looks really bad. You can get a product at automotive stores called Aluminum Brightener. It will take off the oxidization. It'll leave the aluminum a milky white color. You can leave it like that, or polish it up with an aluminum polish.
My advise is to take the box out of your truck to clean it. It is much easier to work on on the ground. Plus, you don't want to be getting all of those chemicals and cleaners on your paint.